Wrapped OS_APPLE around |force_drawing_frame_framebuffer_unflipped_|

This flag is not used for other platforms except MAC so wrapping it in
the Chromium code search has a bug where references for Mac are not
indexed (128985) so it is confusing that code search doesn't show the
four references to this flag inside the #if defined (OS_APPLE) in
gl_renderer.cc.

Bug: n/a
Change-Id: Ife127ec0297e7fccb41e6be68665cde29210b0cc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2488722Reviewed-by: weiliangc <weiliangc@chromium.org>
Commit-Queue: weiliangc <weiliangc@chromium.org>
Auto-Submit: Jun Liu <yjliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819961}

components/viz/service/display/gl_renderer.cc

@@ -3179,8 +3179,10 @@ void GLRenderer::GenerateMipmap() {
 }
 
 bool GLRenderer::FlippedFramebuffer() const {
+#if defined(OS_APPLE)
   if (force_drawing_frame_framebuffer_unflipped_)
     return false;
+#endif
   if (current_frame()->current_render_pass != current_frame()->root_render_pass)
     return true;
   return FlippedRootFramebuffer();

components/viz/service/display/gl_renderer.h

@@ -463,12 +463,14 @@ class VIZ_SERVICE_EXPORT GLRenderer : public DirectRenderer {
   // If true, tints all the composited content to red.
   bool tint_gl_composited_content_ = true;
 
+#if defined(OS_APPLE)
   // The method FlippedFramebuffer determines whether the framebuffer associated
   // with a DrawingFrame is flipped. It makes the assumption that the
   // DrawingFrame is being used as part of a render pass. If a DrawingFrame is
   // not being used as part of a render pass, setting it here forces
   // FlippedFramebuffer to return |true|.
   bool force_drawing_frame_framebuffer_unflipped_ = false;
+#endif
 
   BoundGeometry bound_geometry_;