add drawArraysInstanced to caps

BUG=skia:

Review URL: https://codereview.chromium.org/1202383006
diff --git a/src/gpu/GrCaps.cpp b/src/gpu/GrCaps.cpp
index 900bd65..d6a5470 100644
--- a/src/gpu/GrCaps.cpp
+++ b/src/gpu/GrCaps.cpp
@@ -91,6 +91,7 @@
     fCompressedTexSubImageSupport = false;
     fOversizedStencilSupport = false;
     fTextureBarrierSupport = false;
+    fSupportsInstancedDraws = false;
 
     fUseDrawInsteadOfClear = false;
 
@@ -152,6 +153,7 @@
     r.appendf("Compressed Update Support          : %s\n", gNY[fCompressedTexSubImageSupport]);
     r.appendf("Oversized Stencil Support          : %s\n", gNY[fOversizedStencilSupport]);
     r.appendf("Texture Barrier Support            : %s\n", gNY[fTextureBarrierSupport]);
+    r.appendf("Supports instanced draws           : %s\n", gNY[fSupportsInstancedDraws]);
     r.appendf("Draw Instead of Clear [workaround] : %s\n", gNY[fUseDrawInsteadOfClear]);
     r.appendf("Draw Instead of TexSubImage [workaround] : %s\n",
               gNY[fUseDrawInsteadOfPartialRenderTargetWrite]);