[ChromeDriver] Retry server init in WPT test

Due to occasional ChromeDriver startup timeout on Linux Tests (dbg)
builder, modify run_webdriver_tests.py to retry starting ChromeDriver.

Bug: 931586
Change-Id: I6eb4c0e01f857bff88b26647a9aec67159b898be
Reviewed-on: https://chromium-review.googlesource.com/c/1478332
Commit-Queue: John Chen <johnchen@chromium.org>
Reviewed-by: Caleb Rouleau <crouleau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#633745}
1 file changed