<webview>: Support showing injected scripts in DevTools

This CL plumbs the file URL of the injected script via executeScript so that DevTools can allow debugging of the
script.

BUG=293638
Test=manually, inject a script file via executeScript, open the guest's inspector, go to Sources, click Content scripts, and the script will show up there.

Review URL: https://codereview.chromium.org/23484062

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226882 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226882