Adds glMemoryGlyphIndexArrayNV to the GrGLInterface and begins
detecting support for this method in GrGLPathRendering.

BUG=skia:2939
R=bsalomon@google.com

Author: cdalton@nvidia.com

Review URL: https://codereview.chromium.org/570733002
diff --git a/src/gpu/gl/GrGLInterface.cpp b/src/gpu/gl/GrGLInterface.cpp
index 4c2713d..063de56 100644
--- a/src/gpu/gl/GrGLInterface.cpp
+++ b/src/gpu/gl/GrGLInterface.cpp
@@ -61,6 +61,7 @@
     newInterface->fFunctions.fStencilThenCoverFillPathInstanced = NULL;
     newInterface->fFunctions.fStencilThenCoverStrokePathInstanced = NULL;
     newInterface->fFunctions.fProgramPathFragmentInputGen = NULL;
+    newInterface->fFunctions.fPathMemoryGlyphIndexArray = NULL;
     return newInterface;
 }
 
@@ -485,7 +486,8 @@
                 NULL == fFunctions.fStencilThenCoverStrokePath ||
                 NULL == fFunctions.fStencilThenCoverFillPathInstanced ||
                 NULL == fFunctions.fStencilThenCoverStrokePathInstanced ||
-                NULL == fFunctions.fProgramPathFragmentInputGen) {
+                NULL == fFunctions.fProgramPathFragmentInputGen ||
+                NULL == fFunctions.fPathMemoryGlyphIndexArray) {
                 RETURN_FALSE_INTERFACE
             }
         }