Add WebKit bugzilla/git search to my_activity.py

This is unfortunately imperfect in a number of ways:

* Needs to get info from git commits, so needs a local WebKit git repo, and an explicit option to find it.
* Bugzilla doesn't return nearly as much information as other bug servers, and individual issues would have to be hit (and parsed) to get it.
* Searching for comments includes any issue ever commented on that was changed over the time period requested, so may have false positives.
* Reviews themselves can't be searched from Bugzilla, so can only be detected as "reviews" once landed and "issues" until then.

BUG=158993


Review URL: https://chromiumcodereview.appspot.com/11365078

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@166100 0039d316-1c4b-4281-b951-d872f2087c98