swarming: Support branded mac builds.

BUG=324911
NOTRY=true

Review URL: https://codereview.chromium.org/99893005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238430 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238430
Convert ExtensionFunctionDispatcher from Profile to BrowserContext

Part of the AppShell extensions system refactor project.

BUG=309909
TEST=browser_tests

Review URL: https://codereview.chromium.org/100653003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238431 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238431