`extension.isAllowed???Access` samples

Sample extension for the `extension.isAllowed???Access` APIs, and adding
the functionality to the proxy sample extension.

BUG=76494,74694
TEST=None

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=80329

Review URL: http://codereview.chromium.org/6677108
Patch from Mike West <mkwst@chromium.org>.

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@80331 0039d316-1c4b-4281-b951-d872f2087c98
8 files changed