[Telemetry] Use WaitFor in webpagereplay.py.
[Telemetry] Use WaitFor in webpagereplay.py.

This avoids oversleeping.

BUG=388256

Review URL: https://codereview.chromium.org/464343004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#289604}
git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@289604 0039d316-1c4b-4281-b951-d872f2087c98

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