FBFetch refactor + arm support

BUG=skia:
R=krajcevski@google.com, bsalomon@google.com

Author: joshualitt@chromium.org

Review URL: https://codereview.chromium.org/433603002
diff --git a/src/gpu/gl/GrGLDefines.h b/src/gpu/gl/GrGLDefines.h
index 77b7a15..06dd99d 100644
--- a/src/gpu/gl/GrGLDefines.h
+++ b/src/gpu/gl/GrGLDefines.h
@@ -900,4 +900,8 @@
 // shader stage of <program> (if a fragment stage exists).
 #define GR_GL_FRAGMENT_INPUT                                0x936D
 
+/*  ARM specific define for MSAA support on framebuffer fetch */
+#define GR_GL_FETCH_PER_SAMPLE_ARM                          0x8F65
+
+
 #endif