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Sometimes the screen starts to send updates to JS part to early, when screen
is not registered yet. As a result several messages with context diffs could
be ignored, and JS context became inconsistent with C++ context. I added
'contextReady' message, that is send from JS when context is ready to recieve
updates. Before that the screen handler caches changes in the local context.

BUG=375191

Review URL: https://codereview.chromium.org/581593003

Cr-Commit-Position: refs/heads/master@{#295944}