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When an element is position: fixed and specifies a value for bottom, we
want it to stick to the bottom of the viewport as the URL bar hides (and
the renderer is moved up). However, we missed the special case where
both top and bottom are specified. We can't resize the element in real
time but it's more intuitive to have the element stick to the screen
top.

This also exposed an issue in how we raster the area exposed by the URL
bar. Since we don't resize the viewport layers until the finger is
lifted, the raster code needs to adjust the visible rect by the amount
the URL bar is hidden. This was done by always adding the entire top
controls height in the pending tree. This was guarded by an ANDROID
ifdef which meant the test added here didn't get its benefit and
produced unrastered tiles. I removed the ifdef (URL bar hiding is now
also available on ChromeOS) and improved the expansion to make it more
exactly match the amount of extra visible area.

Bug: 846322
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I02700e08e8254a64405c3adb1a7a123b65468f44
Reviewed-on: https://chromium-review.googlesource.com/1235123
Commit-Queue: David Bokan <bokan@chromium.org>
Reviewed-by: Stefan Zager <szager@chromium.org>
Reviewed-by: vmpstr <vmpstr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594515}