Disable GL_MULTISAMPLE when it's not being used

Updates Gr to disable GL_MULTISAMPLE when drawing to an aliased render
target. This is an important step for NV_framebuffer_mixed_samples,
where just because the color buffer isn't multisampled doesn't mean
other stages in the graphics pipeline aren't. It also removes the need
for the "isLineDraw" workaround that was in GrGLGpu::flushHWAAState.

BUG=skia:

Review URL: https://codereview.chromium.org/1002013006
3 files changed