Remove obsolete enterForceCompositing function

Nothing calls this any more after
https://codereview.chromium.org/323633002/

BUG=none

Review URL: https://codereview.chromium.org/322553006

git-svn-id: svn://svn.chromium.org/blink/trunk@175812 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/public/web/WebWidget.h

@@ -94,11 +94,6 @@ public:
     // and it may result in calls to WebWidgetClient::didInvalidateRect.
     virtual void layout() { }
 
-    // Called to toggle the WebWidget in or out of force compositing mode. This
-    // should be called before paint.
-    // FIXME: Remove this once WebPopuMenuImpl stops overriding it. crbug.com/378029
-    virtual void enterForceCompositingMode(bool enter) { }
-
     // Called to paint the rectangular region within the WebWidget
     // onto the specified canvas at (viewPort.x,viewPort.y). You MUST call
     // Layout before calling this method. It is okay to call paint