Android: disable 2d canvas AA in a different place.

Instead of disabling 2d canvas antialiasing for Android in
content_startup_flags (which can't easily be overridden by content
embedders), disable it in chrome_startup_flags (which disables it for
Chrome and Chromium TestShell on Android, which is what was desired) and
also in shell_main_delegate (which disables it for ContentShell on
Android for the purposes of consistency in manual/automated testing).
Other content embedders (such as android_webview) are now free to
control this flag themselves.

BUG=213238
R=pfeldman@chromium.org, senorblanco@chromium.org, yfriedman@chromium.org

Review URL: https://codereview.chromium.org/26667005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227928 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227928