[I-Spy] Add support for rebaselining expectations from the web UI.

BUG=318865
TEST=unittests included
NOTRY=True

Review URL: https://codereview.chromium.org/106523003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240226 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240226