[system_webview] Add a command line setter tool for system webview.

This CL adds a command line setter tool for system webview very similar
to the ones already present for content shell and android webview.

BUG=

Review URL: https://codereview.chromium.org/1452073002

Cr-Commit-Position: refs/heads/master@{#360250}
1 file changed