Put unpremultiply/dither low bit depth tiles behind runtime flag

After discussion with mariakhomenko@, we decided to leave this feature
enabled despite some expected regressions. This change will allow us
to disable this feature via Finch if any unexpected results appear
as this rolls to stable.

Bug: 825589
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I46506aa8807e1c10f0833e414253d1df11cf1382
Reviewed-on: https://chromium-review.googlesource.com/1014285
Reviewed-by: enne <enne@chromium.org>
Commit-Queue: Eric Karl <ericrk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551095}

• cc/raster/gpu_raster_buffer_provider.cc[diff]
• cc/raster/gpu_raster_buffer_provider.h[diff]
• cc/raster/raster_buffer_provider_perftest.cc[diff]
• cc/raster/raster_buffer_provider_unittest.cc[diff]
• cc/test/layer_tree_pixel_resource_test.cc[diff]
• cc/trees/layer_tree_host_impl.cc[diff]
• cc/trees/layer_tree_host_pixeltest_tiles.cc[diff]
• cc/trees/layer_tree_settings.h[diff]
• content/renderer/gpu/render_widget_compositor.cc[diff]