Implement all resolution change policies for desktop and tab capture.

content/browser/media/capture/web_contents_tracker.h

Index: content/browser/media/capture/web_contents_tracker.h
diff --git a/content/browser/media/capture/web_contents_tracker.h b/content/browser/media/capture/web_contents_tracker.h
index 613c971d44531c1d47aa3d7c125357e59cea21d7..4020a7ee06bf6c395e5f2917a913a59a529c2c6c 100644
--- a/content/browser/media/capture/web_contents_tracker.h
+++ b/content/browser/media/capture/web_contents_tracker.h
@@ -59,6 +59,12 @@ class CONTENT_EXPORT WebContentsTracker
   // Current target.  This must only be called on the UI BrowserThread.
   RenderWidgetHost* GetTargetRenderWidgetHost() const;
 
+  // Set a callback that is run whenever the main frame of the WebContents is
+  // resized.  The callback will be invoked on the same thread that called this
+  // method, which should be the same thread that calls Start()/Stop().  Calling
+  // the Stop() method guarantees the callback will never be invoked again.
+  void SetResizeChangeCallback(const ChangeCallback& callback);
+
  protected:
   friend class base::RefCountedThreadSafe<WebContentsTracker>;
   ~WebContentsTracker() override;
@@ -74,6 +80,11 @@ class CONTENT_EXPORT WebContentsTracker
   // callback is still valid and, if so, runs it.
   void MaybeDoCallback(RenderWidgetHost* rwh);
 
+  // Called on the thread that Start()/Stop() are called on.  Checks whether the
+  // callback is still valid and, if so, runs it to indicate the main frame has
+  // changed in size.
+  void MaybeDoResizeCallback(RenderWidgetHost* rwh);
+
   // Look-up the current WebContents instance associated with the given
   // |render_process_id| and |main_render_frame_id| and begin observing it.
   void StartObservingWebContents(int render_process_id,
@@ -86,6 +97,10 @@ class CONTENT_EXPORT WebContentsTracker
   void RenderFrameHostChanged(RenderFrameHost* old_host,
                               RenderFrameHost* new_host) override;
 
+  // WebContentsObserver override to notify the client that the source size has
+  // changed.
+  void MainFrameWasResized(bool width_changed) override;
+
   // WebContentsObserver override to notify the client that the capture target
   // has been permanently lost.
   void WebContentsDestroyed() override;
@@ -109,6 +124,9 @@ class CONTENT_EXPORT WebContentsTracker
   // This is used to eliminate duplicate callback runs.
   RenderWidgetHost* last_target_;
 
+  // Callback to run when the target RenderWidgetHost has resized.
+  ChangeCallback resize_callback_;
+
   DISALLOW_COPY_AND_ASSIGN(WebContentsTracker);
 };