Add Page.stopLoading to devtools and --timeout switch to headless_shell

Some pages don't load in a reasonable timeframe so allow users to
cause the load to stop after a given timeout.  We implement this via a
new Page.stopLoading devtools command.

The new switch is --timeout=milliseconds (e.g. --timeout=5000)

BUG=546953
R=dgozman@chromium.org, skyostil@chromium.org

Review URL: https://codereview.chromium.org/2525903003 .

Cr-Commit-Position: refs/heads/master@{#434480}
9 files changed