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If user data needs to be saved after shut down, call Chrome DevTools Protocol
browser.close. If browser.close failed, call KillProcess.
Do not call closeWebview when only one window left.

Bug: chromedriver:3370
Change-Id: I9022500332e0f971d9535ae2117c88f577144c5d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2283057Reviewed-by: John Chen <johnchen@chromium.org>
Commit-Queue: Shengfa Lin <shengfa@google.com>
Cr-Commit-Position: refs/heads/master@{#786344}