Avoid null check on LayerTreeHost, stop passing stack pointer to heap

The LatencyInfoSwapPromiseMonitors do not need to be heap allocated,
they are destroyed within the same stack frame where they are created
but they are put on the heap because the LayerTreeHost is treated as
being possibly null.

The LayerTreeHost would only be null if the RenderWidget is closing,
and an IPC is received on the handler through some other channel.
Instead, don't call to the handler from RenderWidget::HandleInputEvent
if the RenderWidget is closing. Then no null checks are needed, and
the LatencyInfoSwapPromiseMonitors can be created on the stack.

Also narrow their lifetime to just be around the method call that
they are monitoring, for clarity.

R=avi@chromium.org

Bug: 994926
Change-Id: I47a04f248479a6266fba673ea0a2e98ce1671e66
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1812915Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#698489}

content/renderer/input/render_widget_input_handler.cc

@@ -14,7 +14,6 @@
 #include "build/build_config.h"
 #include "cc/paint/element_id.h"
 #include "cc/trees/latency_info_swap_promise_monitor.h"
-#include "cc/trees/swap_promise_monitor.h"
 #include "components/viz/common/surfaces/frame_sink_id.h"
 #include "content/common/input/input_event_ack.h"
 #include "content/public/common/content_switches.h"
@@ -365,17 +364,9 @@ void RenderWidgetInputHandler::HandleInputEvent(
   if (!start_time.is_null())
     LogInputEventLatencyUma(input_event, start_time);
 
-  std::unique_ptr<cc::SwapPromiseMonitor> latency_info_swap_promise_monitor;
   ui::LatencyInfo swap_latency_info(latency_info);
-
   swap_latency_info.AddLatencyNumber(
       ui::LatencyComponentType::INPUT_EVENT_LATENCY_RENDERER_MAIN_COMPONENT);
-  if (widget_->layer_tree_host()) {
-    latency_info_swap_promise_monitor =
-        std::make_unique<cc::LatencyInfoSwapPromiseMonitor>(
-            &swap_latency_info,
-            widget_->layer_tree_host()->GetSwapPromiseManager(), nullptr);
-  }
 
   bool prevent_default = false;
   bool show_virtual_keyboard_for_mouse = false;
@@ -385,7 +376,13 @@ void RenderWidgetInputHandler::HandleInputEvent(
     TRACE_EVENT2("renderer", "HandleMouseMove", "x",
                  mouse_event.PositionInWidget().x, "y",
                  mouse_event.PositionInWidget().y);
-    prevent_default = delegate_->WillHandleMouseEvent(mouse_event);
+
+    {
+      cc::LatencyInfoSwapPromiseMonitor swap_promise_monitor(
+          &swap_latency_info,
+          widget_->layer_tree_host()->GetSwapPromiseManager(), nullptr);
+      prevent_default = delegate_->WillHandleMouseEvent(mouse_event);
+    }
 
     // Reset the last known cursor if mouse has left this widget. So next
     // time that the mouse enters we always set the cursor accordingly.
@@ -667,14 +664,6 @@ void RenderWidgetInputHandler::HandleInjectedScrollGestures(
       }
     }
 
-    std::unique_ptr<cc::SwapPromiseMonitor> swap_promise_monitor;
-    if (widget_->layer_tree_host()) {
-      swap_promise_monitor =
-          std::make_unique<cc::LatencyInfoSwapPromiseMonitor>(
-              &scrollbar_latency_info,
-              widget_->layer_tree_host()->GetSwapPromiseManager(), nullptr);
-    }
-
     std::unique_ptr<WebGestureEvent> gesture_event =
         ui::GenerateInjectedScrollGesture(
             params.type, input_event.TimeStamp(), params.device, position,
@@ -687,8 +676,13 @@ void RenderWidgetInputHandler::HandleInjectedScrollGestures(
       last_injected_gesture_was_begin_ = false;
     }
 
-    widget_->GetWebWidget()->HandleInputEvent(
-        blink::WebCoalescedInputEvent(*gesture_event.get()));
+    {
+      cc::LatencyInfoSwapPromiseMonitor swap_promise_monitor(
+          &scrollbar_latency_info,
+          widget_->layer_tree_host()->GetSwapPromiseManager(), nullptr);
+      widget_->GetWebWidget()->HandleInputEvent(
+          blink::WebCoalescedInputEvent(*gesture_event.get()));
+    }
   }
 }
 

content/renderer/render_widget.cc

@@ -1078,6 +1078,8 @@ bool RenderWidget::HandleInputEvent(
     const blink::WebCoalescedInputEvent& input_event,
     const ui::LatencyInfo& latency_info,
     HandledEventCallback callback) {
+  if (closing_)
+    return false;
   if (IsUndeadOrProvisional())
     return false;
   input_handler_->HandleInputEvent(input_event, latency_info,