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The original CL was later reverted (CL crrev.com/2068953003) due to
frequent failures of the SAMLPolicyTest.SAMLInterstitialNext test.

This follow-up CL should resolve the tests flakiness.

The problem with the original CL was that it could introduce raising the
"ready" event too early, so that the test code could start working when
the webview was still pointing to the blank page.

BUG=613245,620353
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2082533002
Cr-Commit-Position: refs/heads/master@{#400703}