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Sometimes, especially on Windows, initial creation of the content_shell window
takes multiple seconds to complete. Tests then result in flaky timeouts. This change
ensures the window is initialized before logging #READY. The run_web_tests harness has
code to not count the time between launching content_shell and logging #READY.

Bug: 953991
Change-Id: I50c2449e204cdb2d66996bcb4c553a2f083604f7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1584476
Commit-Queue: Austin Eng <enga@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658594}