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There were couple issues that were fixed:
1. If oninput listener fires before the guest is requested to "wait
for oninput", we would timeout. This is taken care of by a flag.
2. TextInputClient::SetCompositionText() call sends an IPC to the
renderer/, if the next JavaScript step runs before the IPC is sent
then that would fail the test. This is taken care of by sending
executing empty JavaScript code on the embedder.

BUG=387956
Test=Internal test only change, nothing visible.

Review URL: https://codereview.chromium.org/532883003

Cr-Commit-Position: refs/heads/master@{#293341}