[Mac] Do not show the 'Make X the default browser' button on Canary.
For reasons that appear largely historical, the code path for determining the
default-allowing state in the settings UI is special-cased on Mac. This removes
that and just uses the cross-platform logic, which doesn't enable the button for
Canary.
BUG=370548
TEST=Put KSChannelID=canary in the Info.plist (or just run Canary) and chrome://settings button for the default browser is not present.
R=jhawkins@chromium.org
Review URL: https://codereview.chromium.org/
490523004
Cr-Commit-Position: refs/heads/master@{#290642}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@290642 0039d316-1c4b-4281-b951-d872f2087c98