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Register a context loss callback with the GLES2Implementation to find
out about lost context. Implement AddObserver() and RemoveObserver() for
VizProcessContextProvider to notify the observers on context loss.

viz::Display is already registered as the context loss observer for the
ContextProvider. It gets notified and calls out to DisplayClient.
Implement a handler for DisplayOutputSurfaceLost() so the root
CompositorFrameSink shuts down the display on context loss. The client
will see a connection error and reinitialize the display.

Also make VizProcessContextProvider log UMA on context loss. This
requires moving ContextLossReason enum somewhere visible to
VizProcessContextProvider and adding a new histogram for
GPU.ContextLost.DisplayCompositor.

Bug: 859068
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: I4c1fb6f00a893003e9b65dde1a7c5802a8f27692
Reviewed-on: https://chromium-review.googlesource.com/1126482Reviewed-by: Antoine Labour <piman@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Robert Kaplow (slow) <rkaplow@chromium.org>
Commit-Queue: kylechar <kylechar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575016}