Surface Synchronization: Don't ship RenderFrameMetadata on change in scroll

We don't need to ship updated RenderFrameMetadata on change in root scroll
offset. Shipping RenderFrameMetadata is expensive and so we should minimize
shipping them to the browser process as it also involves synchronization
with Viz.

We initially decided to plumb this value via RenderFrameMetadata in order
to properly support the overscroll glow on Android but this turned out to
be too expensive and so we are now investigating ways to compute the
overscroll glow at least partially in the renderer.

Bug: 672962, 855473
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I139f773eaf673785fec04c9f989abb36e0941ad9
Reviewed-on: https://chromium-review.googlesource.com/1113799
Commit-Queue: Fady Samuel <fsamuel@chromium.org>
Reviewed-by: Saman Sami <samans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570153}

cc/trees/render_frame_metadata.cc

@@ -30,7 +30,6 @@ bool RenderFrameMetadata::HasAlwaysUpdateMetadataChanged(
          rfm1.top_controls_shown_ratio != rfm2.top_controls_shown_ratio ||
          rfm1.bottom_controls_height != rfm2.bottom_controls_height ||
          rfm1.bottom_controls_shown_ratio != rfm2.bottom_controls_shown_ratio ||
-         rfm1.root_scroll_offset != rfm2.root_scroll_offset ||
          rfm1.min_page_scale_factor != rfm2.min_page_scale_factor ||
          rfm1.max_page_scale_factor != rfm2.max_page_scale_factor ||
          rfm1.root_overflow_y_hidden != rfm2.root_overflow_y_hidden ||