Clarify EmulationHandler early outs, null checks, and frame relationship

The EmulationHandler only enables touch or device emulation on the main
frame. Currently the touch handler is explicit about this, which was
added a few months after the check for being a local root. That change
should obsolete the local root check (main frame is a subset of local
root frames), though both remain in the code. Remove the local root
comparison.

The device emulation path still only checks for local root, but could
be changed to main frame also for more clarity.

The GetWebContents() method returns null only if the host_ is null, so
instead of checking both of these for null, only call GetWebContents()
when |host_| is not null, and change methods to check for |host_| in
places that they checked for GetWebContents().

The GetRenderWidgetHost() on RenderFrameHostImpl never returns null,
as per its comment:

  // Returns the RenderWidgetHostImpl attached to this frame or the nearest
  // ancestor frame, which could potentially be the root. For most input
  // and rendering related purposes, GetView() should be preferred and
  // RenderWidgetHostViewBase methods used. GetRenderWidgetHost() will not
  // return a nullptr, whereas GetView() potentially will (for instance,
  // after a renderer crash).
  //
  // This method crashes if this RenderFrameHostImpl does not own a
  // a RenderWidgetHost and nor does any of its ancestors. That would
  // typically mean that the frame has been detached from the frame tree.
  virtual RenderWidgetHostImpl* GetRenderWidgetHost();

R=yangguo@chromium.org

Bug: 1006052
Change-Id: Ia43c2fb04de343f7c6e2c7cd1600a9bdc1a14908
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1815735
Auto-Submit: danakj <danakj@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#698840}

content/browser/devtools/protocol/emulation_handler.cc

@@ -103,7 +103,7 @@ Response EmulationHandler::Disable() {
 
 Response EmulationHandler::SetGeolocationOverride(
     Maybe<double> latitude, Maybe<double> longitude, Maybe<double> accuracy) {
-  if (!GetWebContents())
+  if (!host_)
     return Response::InternalError();
 
   auto* geolocation_context = GetWebContents()->GetGeolocationContext();
@@ -126,7 +126,7 @@ Response EmulationHandler::SetGeolocationOverride(
 }
 
 Response EmulationHandler::ClearGeolocationOverride() {
-  if (!GetWebContents())
+  if (!host_)
     return Response::InternalError();
 
   auto* geolocation_context = GetWebContents()->GetGeolocationContext();
@@ -148,10 +148,11 @@ Response EmulationHandler::CanEmulate(bool* result) {
   *result = false;
 #else
   *result = true;
-  if (WebContentsImpl* web_contents = GetWebContents())
-    *result &= !web_contents->GetVisibleURL().SchemeIs(kChromeDevToolsScheme);
-  if (host_ && host_->GetRenderWidgetHost())
-    *result &= !host_->GetRenderWidgetHost()->auto_resize_enabled();
+  if (host_) {
+    if (GetWebContents()->GetVisibleURL().SchemeIs(kChromeDevToolsScheme) ||
+        host_->GetRenderWidgetHost()->auto_resize_enabled())
+      *result = false;
+  }
 #endif  // defined(OS_ANDROID)
   return Response::OK();
 }
@@ -173,9 +174,7 @@ Response EmulationHandler::SetDeviceMetricsOverride(
   const static double max_scale = 10;
   const static int max_orientation_angle = 360;
 
-  RenderWidgetHostImpl* widget_host =
-      host_ ? host_->GetRenderWidgetHost() : nullptr;
-  if (!widget_host)
+  if (!host_)
     return Response::Error("Target does not support metrics override");
 
   if (screen_width.fromMaybe(0) < 0 || screen_height.fromMaybe(0) < 0 ||
@@ -243,7 +242,7 @@ Response EmulationHandler::SetDeviceMetricsOverride(
     params.viewport_offset.y = viewport.fromJust()->GetY();
 
     ScreenInfo screen_info;
-    widget_host->GetScreenInfo(&screen_info);
+    host_->GetRenderWidgetHost()->GetScreenInfo(&screen_info);
     double dpfactor = device_scale_factor ? device_scale_factor /
                                                 screen_info.device_scale_factor
                                           : 1;
@@ -289,17 +288,16 @@ Response EmulationHandler::SetDeviceMetricsOverride(
 Response EmulationHandler::ClearDeviceMetricsOverride() {
   if (!device_emulation_enabled_)
     return Response::OK();
-  if (GetWebContents())
-    GetWebContents()->ClearDeviceEmulationSize();
-  else
+  if (!host_)
     return Response::Error("Can't find the associated web contents");
+  GetWebContents()->ClearDeviceEmulationSize();
   device_emulation_enabled_ = false;
   device_emulation_params_ = blink::WebDeviceEmulationParams();
   UpdateDeviceEmulationState();
   // Renderer should answer after emulation was disabled, so that the response
   // is only sent to the client once updates were applied.
   // Unless the renderer has crashed.
-  if (GetWebContents() && GetWebContents()->IsCrashed())
+  if (GetWebContents()->IsCrashed())
     return Response::OK();
   return Response::FallThrough();
 }
@@ -308,11 +306,9 @@ Response EmulationHandler::SetVisibleSize(int width, int height) {
   if (width < 0 || height < 0)
     return Response::InvalidParams("Width and height must be non-negative");
 
-  if (GetWebContents())
-    GetWebContents()->SetDeviceEmulationSize(gfx::Size(width, height));
-  else
+  if (!host_)
     return Response::Error("Can't find the associated web contents");
-
+  GetWebContents()->SetDeviceEmulationSize(gfx::Size(width, height));
   return Response::OK();
 }
 
@@ -346,17 +342,13 @@ void EmulationHandler::SetDeviceEmulationParams(
 }
 
 WebContentsImpl* EmulationHandler::GetWebContents() {
-  return host_ ?
-      static_cast<WebContentsImpl*>(WebContents::FromRenderFrameHost(host_)) :
-      nullptr;
+  DCHECK(host_);  // Only call if |host_| is set.
+  return static_cast<WebContentsImpl*>(WebContents::FromRenderFrameHost(host_));
 }
 
 void EmulationHandler::UpdateTouchEventEmulationState() {
-  if (!host_ || !host_->GetRenderWidgetHost())
-    return;
-  if (host_->GetParent() && !host_->IsCrossProcessSubframe())
+  if (!host_)
     return;
-
   // We only have a single TouchEmulator for all frames, so let the main frame's
   // EmulationHandler enable/disable it.
   if (!host_->frame_tree_node()->IsMainFrame())
@@ -373,17 +365,16 @@ void EmulationHandler::UpdateTouchEventEmulationState() {
     if (auto* touch_emulator = host_->GetRenderWidgetHost()->GetTouchEmulator())
       touch_emulator->Disable();
   }
-  if (GetWebContents()) {
-    GetWebContents()->SetForceDisableOverscrollContent(
-        touch_emulation_enabled_);
-  }
+  GetWebContents()->SetForceDisableOverscrollContent(touch_emulation_enabled_);
 }
 
 void EmulationHandler::UpdateDeviceEmulationState() {
-  if (!host_ || !host_->GetRenderWidgetHost())
+  if (!host_)
     return;
-  if (host_->GetParent() && !host_->IsCrossProcessSubframe())
+  // Device emulation only happens on the main frame.
+  if (!host_->frame_tree_node()->IsMainFrame())
     return;
+
   // TODO(eseckler): Once we change this to mojo, we should wait for an ack to
   // these messages from the renderer. The renderer should send the ack once the
   // emulation params were applied. That way, we can avoid having to handle