Previously, we weren't scheduling a visual update when our surface was created,
so we'd just continue to show black until the author requested a new animation
frame. After this CL, we schedule a visual update as soon as our surface is
created.

As part of this change, I've removed knowledge of the GPU delegate from
PlatformView. Now, all the calls from PlatformView to the GPU system bounce
through the UI delegate, which serializes the commands with other commands from
the UI engine to the GPU rasterizer.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/974483004