Make it possible to toggle MSAA for ES (EXT_multisample_compatibility)

Make it possible to turn MSAA on and off for OpenGL ES, too.
Use EXT_multisample_compatibility.

The extension adds token:
        MULTISAMPLE_EXT                             0x809D

The token can be supplied to glEnable/glDisable/glGetInteger.

This is needed in order to support mixed samples in OpenGL ES. This is
important for path rendering Chromium command buffer integration.

Review URL: https://codereview.chromium.org/1270533002
diff --git a/src/gpu/gl/GrGLCaps.cpp b/src/gpu/gl/GrGLCaps.cpp
index b0f6f2f..a064b6a 100644
--- a/src/gpu/gl/GrGLCaps.cpp
+++ b/src/gpu/gl/GrGLCaps.cpp
@@ -291,7 +291,7 @@
     if (kGL_GrGLStandard == standard) {
         fMultisampleDisableSupport = true;
     } else {
-        fMultisampleDisableSupport = false;
+        fMultisampleDisableSupport = ctxInfo.hasExtension("GL_EXT_multisample_compatibility");
     }
 
     if (kGL_GrGLStandard == standard && version >= GR_GL_VER(3, 0)) {