Telemetry support for iframes.

* Add methods EvaluateJavaScriptInContext,
ExecuteJavaScriptInContext.
* Backend classes take an additional context_id argument, which refers to the iframe.
* WebContents has an EnableAllContexts method, to enable access to iframes.
* Add tests testIFrame, and unit test data files host.html, iframe*.html.

BUG=237032
TEST=unittest

Review URL: https://codereview.chromium.org/183863007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255621 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255621
Implement File::WriteAtCurrentPos for Windows

BUG=145873, 322664
R=cpu@chromium.org

Review URL: https://codereview.chromium.org/183333004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255622 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255622