Bug 1576499 - Change where aClipRectOut is set, in two BeginFrame implementations. r=mattwoodrow

There are three circumstances that make these transformations equivalent:
 - aClipRectIn and aClipRectOut are never both non-null.
 - The value of *aClipRectOut is ignored if *aRenderBoundsOut is empty.
 - aRenderBounds.TopLeft() is always (0, 0) because the ClientLayerManager
   calls targetBounds.MoveTo(0, 0) on the rect that eventually becomes
   aRenderBounds.
   (targetBounds -> Transaction::mTargetBounds -> TargetConfig::naturalBounds()
    -> LayerManagerComposite::mRenderBounds)

Depends on D43367

Differential Revision: https://phabricator.services.mozilla.com/D43368

--HG--
extra : moz-landing-system : lando

gfx/layers/basic/BasicCompositor.cpp

@@ -1036,14 +1036,14 @@ void BasicCompositor::BeginFrame(const nsIntRegion& aInvalidRegion,
   if (aRenderBoundsOut) {
     *aRenderBoundsOut = rect;
   }
+  if (aClipRectOut) {
+    *aClipRectOut = rect;
+  }
 
   if (aClipRectIn) {
     mRenderTarget->mDrawTarget->PushClipRect(Rect(*aClipRectIn));
   } else {
     mRenderTarget->mDrawTarget->PushClipRect(Rect(rect));
-    if (aClipRectOut) {
-      *aClipRectOut = rect;
-    }
   }
 }
 

gfx/layers/opengl/CompositorOGL.cpp

@@ -889,6 +889,9 @@ void CompositorOGL::BeginFrame(const nsIntRegion& aInvalidRegion,
   if (aRenderBoundsOut) {
     *aRenderBoundsOut = rect;
   }
+  if (aClipRectOut) {
+    *aClipRectOut = rect;
+  }
 
   // We can't draw anything to something with no area
   // so just return
@@ -948,10 +951,6 @@ void CompositorOGL::BeginFrame(const nsIntRegion& aInvalidRegion,
   SetRenderTarget(rt);
   mWindowRenderTarget = mCurrentRenderTarget;
 
-  if (aClipRectOut && !aClipRectIn) {
-    *aClipRectOut = IntRect(IntPoint(), rect.Size());
-  }
-
 #if defined(MOZ_WIDGET_ANDROID)
   if ((mSurfaceOrigin.x > 0) || (mSurfaceOrigin.y > 0)) {
     mGLContext->fClearColor(