Add the test_platform factory property to iOS try builders.
Add the test_platform factory property to iOS try builders.

The test_platform fp is used by build/scripts/slave/runtest.py to distinguish between mac and ios tests.

Review URL: https://chromiumcodereview.appspot.com/10960031

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@158562 0039d316-1c4b-4281-b951-d872f2087c98

eunchong/build,eunchong/build,eunchong/build,eunchong/build