Revert 255621 "Telemetry support for iframes."

> 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

BUG=350568
TBR=achuith@chromium.org

Review URL: https://codereview.chromium.org/191643002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255715 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255715