This patch adds GetMultismaplefvANGLE and SampleMaskiANGLE api.

TEST=TextureMultisampleTest.CheckSamplePositions*
TEST=NegativeTextureMultisampleTest.Negtive*

BUG=angleproject:2275

Change-Id: Ib1e8b856059229bbe88328a28cf2a892f4673214
Reviewed-on: https://chromium-review.googlesource.com/c/1319252
Commit-Queue: Yizhou Jiang <yizhou.jiang@intel.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>