Plumb an overscroll callback form the compositor to the Android UI thread.

Plumb an overscroll callback form the compositor to the Android UI thread.

This is a first step towards implementing overscroll on Android.

BUG=

[mkosiba (inactive), 2013-01-30 14:46:23]

Hey!
    PTAL. Also, feel free to suggest better naming :)

https://codereview.chromium.org/12077046/diff/1/cc/layer_tree_host_impl.cc
File cc/layer_tree_host_impl.cc (right):

https://codereview.chromium.org/12077046/diff/1/cc/layer_tree_host_impl.cc#ne...
cc/layer_tree_host_impl.cc:773: m_pendingScrollDelta = gfx::Vector2dF();
where would be a good place to reset is variable? I tried doing that in
OnSendFrameToParentCompositorAck, didDrawAllLayers and processScrollDeltas but
all of those seem to happen too early.

https://codereview.chromium.org/12077046/diff/1/cc/layer_tree_host_impl.cc#ne...
cc/layer_tree_host_impl.cc:1372: if (didScroll ||
!gfx::ToFlooredVector2d(overScrollDelta).IsZero()) {
this makes us draw frames even though the contents of those frames would not
change. is there a simple change I could do to optimize that away or is it not
worth it?

[aelias_OOO_until_Jul13, 2013-01-30 18:02:37]

General comment, I think you'll need to gather different information for flings
reaching the limit, I don't think the scroll information is enough.

[aelias_OOO_until_Jul13, 2013-01-30 18:02:48]

https://codereview.chromium.org/12077046/diff/1/cc/compositor_frame_metadata.h
File cc/compositor_frame_metadata.h (right):

https://codereview.chromium.org/12077046/diff/1/cc/compositor_frame_metadata....
cc/compositor_frame_metadata.h:37: // The amount scroll delta that was unused.
Mention this is for Android overscroll.

https://codereview.chromium.org/12077046/diff/1/cc/layer_tree_host_impl.cc
File cc/layer_tree_host_impl.cc (right):

https://codereview.chromium.org/12077046/diff/1/cc/layer_tree_host_impl.cc#ne...
cc/layer_tree_host_impl.cc:773: m_pendingScrollDelta = gfx::Vector2dF();
I guess it's okay to do it here.  You can remove the const from the method.

https://codereview.chromium.org/12077046/diff/1/cc/layer_tree_host_impl.cc#ne...
cc/layer_tree_host_impl.cc:1329: gfx::Vector2dF overScrollDelta = scrollDelta;
Call this "unused" for consistency.

https://codereview.chromium.org/12077046/diff/1/cc/layer_tree_host_impl.cc#ne...
cc/layer_tree_host_impl.cc:1372: if (didScroll ||
!gfx::ToFlooredVector2d(overScrollDelta).IsZero()) {
I think the redraw itself is okay but let's avoid the commit and tree priority
update, you shouldn't need those.

https://codereview.chromium.org/12077046/diff/1/cc/layer_tree_host_impl.h
File cc/layer_tree_host_impl.h (right):

https://codereview.chromium.org/12077046/diff/1/cc/layer_tree_host_impl.h#new...
cc/layer_tree_host_impl.h:335: mutable gfx::Vector2dF m_pendingScrollDelta;
Call this "unused" for consistency, and shouldn't be mutable.

https://codereview.chromium.org/12077046/diff/1/content/common/cc_messages.h
File content/common/cc_messages.h (right):

https://codereview.chromium.org/12077046/diff/1/content/common/cc_messages.h#...
content/common/cc_messages.h:230: IPC_STRUCT_TRAITS_MEMBER(unused_scroll_delta)
Place this at the end.

https://codereview.chromium.org/12077046/diff/1/content/common/view_messages.h
File content/common/view_messages.h (right):

https://codereview.chromium.org/12077046/diff/1/content/common/view_messages....
content/common/view_messages.h:699:
IPC_STRUCT_BEGIN(ViewHostMsg_UpdateFrameInfo_Params)
ViewHostMsg_UpdateFrameInfo is going to get deleted after
--enable-compositor-frame-message is set, so I recommend not doing this.  Just
leave them in separate values in the browser process calls and don't touch
UpdateFrameInfo in the renderer.