Add a string to user agent to differentiate webview from clank.

Add a string representing webview to the user agent string for webview
to be able to differentiate between webview and clank.

BUG=461383

Review URL: https://codereview.chromium.org/1035973002

Cr-Commit-Position: refs/heads/master@{#322834}
5 files changed