cc: Remove support for non-async compositor worker mode.

This change removes compositor worker GLES2 ContextGroup sharing in the Browser
Compositor, and makes async compositor worker mode the only supported mode.

Bug: 796734,757607
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.android:android_optional_gpu_tests_rel
Change-Id: I2d3713b8948b5db2d9a437733f4bf58309f2f36e
Reviewed-on: https://chromium-review.googlesource.com/838243
Commit-Queue: Victor Miura <vmiura@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Reviewed-by: Sunny Sachanandani <sunnyps@chromium.org>
Cr-Commit-Position: refs/heads/master@{#525598}

• cc/raster/gpu_raster_buffer_provider.cc[diff]
• cc/raster/gpu_raster_buffer_provider.h[diff]
• cc/raster/one_copy_raster_buffer_provider.cc[diff]
• cc/raster/one_copy_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_settings.h[diff]
• content/browser/compositor/gpu_process_transport_factory.cc[diff]
• content/browser/compositor/gpu_process_transport_factory.h[diff]
• content/browser/compositor/viz_process_transport_factory.cc[diff]
• content/browser/compositor/viz_process_transport_factory.h[diff]
• content/browser/gpu/compositor_util.cc[diff]
• content/browser/gpu/compositor_util.h[diff]
• content/browser/renderer_host/render_process_host_impl.cc[diff]
• content/public/common/content_switches.cc[diff]
• content/public/common/content_switches.h[diff]
• content/renderer/gpu/compositor_dependencies.h[diff]
• content/renderer/gpu/render_widget_compositor.cc[diff]
• content/renderer/render_thread_impl.cc[diff]
• content/renderer/render_thread_impl.h[diff]
• content/test/fake_compositor_dependencies.cc[diff]
• content/test/fake_compositor_dependencies.h[diff]