Bug 1137944 - Add a pref to control plugin window hiding. r=roc

gfx/thebes/gfxPrefs.h

@@ -240,6 +240,7 @@ private:
   DECL_GFX_PREF(Live, "gfx.direct2d.use1_1",                   Direct2DUse1_1, bool, false);
   DECL_GFX_PREF(Live, "gfx.direct2d.allow1_0",                 Direct2DAllow1_0, bool, false);
   DECL_GFX_PREF(Live, "gfx.draw-color-bars",                   CompositorDrawColorBars, bool, false);
+  DECL_GFX_PREF(Once, "gfx.e10s.hide-plugins-for-scroll",      HidePluginsForScroll, bool, true);
   DECL_GFX_PREF(Once, "gfx.font_rendering.directwrite.force-enabled", DirectWriteFontRenderingForceEnabled, bool, false);
   DECL_GFX_PREF(Live, "gfx.gralloc.fence-with-readpixels",     GrallocFenceWithReadPixels, bool, false);
   DECL_GFX_PREF(Live, "gfx.layerscope.enabled",                LayerScopeEnabled, bool, false);

layout/generic/nsGfxScrollFrame.cpp

@@ -1997,6 +1997,9 @@ void
 ScrollFrameHelper::NotifyPluginFrames(AsyncScrollEventType aEvent)
 {
 #if defined(XP_WIN) || defined(MOZ_WIDGET_GTK)
+  if (!gfxPrefs::HidePluginsForScroll()) {
+    return;
+  }
   if (XRE_IsContentProcess()) {
     if (aEvent != mAsyncScrollEvent) {
       nsPresContext* presContext = mOuter->PresContext();