gpu: Use virtualized context with MailboxManagerSync
gpu: Use virtualized context with MailboxManagerSync

MailboxManagerSync synchronization correctness depends on having only a
single underlying context, since EGLFence only guarantees
synchronization in context the fence is created.

Eg this sequence is broken:
Context 1: Modify Texture A.
Context 2: Insert sync point unrelated to Texture A. This causes
PushTextureUpdates for Texture A, modification of Texture A from Context
1 is not guaranteed to have completed at this point.
Context 1: Insert sync point. TextureUpdate for Texture A is skipped due
to version check.

BUG=510243

Review URL: https://codereview.chromium.org/1430303002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#358678}

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