Prevent STATUS_ACCESS_VIOLATION in inspector overlay

Prior to a recent change [1], `InspectorOverlayAgent::SetInspectTool`
would call `InspectorOverlayAgent::LoadFrameForTool` which assigns to
`overlay_page_`. With the patch, that no longer happened, resulting in
`STATUS_ACCESS_VIOLATION` “oh snap” errors when using the “capture area
screenshot” functionality in DevTools.

This patch moves the “is the inspector agent enabled” check around so
that we don’t hit any `nullptr`s while still getting the desired
behavior of not rendering inspector overlays while taking screenshots.

[1]: https://chromium-review.googlesource.com/c/chromium/src/+/2235553

Bug: chromium:1091037, chromium:1122038
Change-Id: I36ab8b883f1e643bcc89ebaed4063cc1409ee466
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2387560Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Commit-Queue: Mathias Bynens <mathias@chromium.org>
Cr-Commit-Position: refs/heads/master@{#803417}

third_party/blink/renderer/core/inspector/inspector_overlay_agent.cc

@@ -1372,13 +1372,13 @@ void InspectorOverlayAgent::EnsureEnableFrameOverlay() {
 void InspectorOverlayAgent::SetInspectTool(InspectTool* inspect_tool) {
   LocalFrameView* view = frame_impl_->GetFrameView();
   LocalFrame* frame = GetFrame();
-  if (!view || !frame || !enabled_.Get())
+  if (!view || !frame)
     return;
 
   if (inspect_tool_)
     inspect_tool_->Dispose();
 
-  if (inspect_tool) {
+  if (inspect_tool && enabled_.Get()) {
     inspect_tool_ = inspect_tool;
     LoadFrameForTool(inspect_tool->GetDataResourceId());
     EnsureEnableFrameOverlay();