Enable image decoder tests on Android

They were excluded on Android because the data files are not pushed
onto the device. Added data file dependency in WebKitUnitTests.isolate.

BUG=none

Review URL: https://chromiumcodereview.appspot.com/22903033

git-svn-id: svn://svn.chromium.org/blink/trunk@156646 bbb929c8-8fbe-4397-9dbb-9b2b20218538