Distance field path optimizations and clean up.

Adds the following:
- Use cached geometry processor rather than recreating all the time.
- Use context's quad index buffer.

Review URL: https://codereview.chromium.org/683923002
diff --git a/src/gpu/GrDistanceFieldTextContext.cpp b/src/gpu/GrDistanceFieldTextContext.cpp
index 66ec123..118565a 100755
--- a/src/gpu/GrDistanceFieldTextContext.cpp
+++ b/src/gpu/GrDistanceFieldTextContext.cpp
@@ -74,7 +74,7 @@
 
     fEffectTextureUniqueID = SK_InvalidUniqueID;
     fEffectColor = GrColor_ILLEGAL;
-    fEffectFlags = 0;
+    fEffectFlags = kInvalid_DistanceFieldEffectFlag;
 
     fVertices = NULL;
     fCurrVertex = 0;