Fixed position layer counter-scroll with main thread scroll offset fractional part

cc/layers/layer.cc

Index: cc/layers/layer.cc
diff --git a/cc/layers/layer.cc b/cc/layers/layer.cc
index feb23fa8d84f4e800c135e606335a47d103d03ff..219e63f3d9fc5fe294d91849f7721b5845d1658e 100644
--- a/cc/layers/layer.cc
+++ b/cc/layers/layer.cc
@@ -669,6 +669,8 @@ void Layer::SetScrollOffset(const gfx::ScrollOffset& scroll_offset) {
   if (scroll_offset_ == scroll_offset)
     return;
   scroll_offset_ = scroll_offset;
+  scroll_offset_fractional_part_ = scroll_offset.DeltaFrom(

[aelias_OOO_until_Jul13, 2015/01/28 02:04:00]

I see Dana made the comment on your doc "This seems magical in that blink
positions things at integers since it knows cc will happen to only use the
integer parts of the things it gave. It should have to be more explicit."  In
keeping with that, how about a new API "SetScrollOffsetFractionalPart" to be
called from Blink and leave SetScrollOffset unchanged?

[Yufeng Shen (Slow to review), 2015/01/28 04:49:23]

would an API that takes 2 parameters be better ?
e.g. SetScrollOffset(scroll_offset_integer_part, scroll_offset_fractional_part)

so the caller won't forget to call one without the other.

[aelias_OOO_until_Jul13, 2015/01/28 05:50:29]

I'd rather keep them separate, it makes sense not to call the fractional one if
the fixed-position problem in Blink is fixed at some point.

[Yufeng Shen (Slow to review), 2015/01/28 22:28:09]

Done.

+      gfx::ScrollOffset(gfx::ScrollOffsetToFlooredVector2d(scroll_offset)));
   SetNeedsCommit();
 }
 
@@ -981,6 +983,7 @@ void Layer::PushPropertiesTo(LayerImpl* layer) {
     }
     layer->SetSentScrollDelta(gfx::Vector2dF());
   }
+  layer->SetMainScrollOffsetFractionalPart(scroll_offset_fractional_part_);
 
   // Wrap the copy_requests_ in a PostTask to the main thread.
   ScopedPtrVector<CopyOutputRequest> main_thread_copy_requests;