Don't draw if SkShader::asNewFragmentProcessor fails.

BUG=chromium:473156

Review URL: https://codereview.chromium.org/1089063002
diff --git a/src/gpu/GrTextContext.cpp b/src/gpu/GrTextContext.cpp
index 3218710..6153f3b 100644
--- a/src/gpu/GrTextContext.cpp
+++ b/src/gpu/GrTextContext.cpp
@@ -116,7 +116,9 @@
         runPaint.setFlags(fGpuDevice->filterTextFlags(runPaint));
 
         GrPaint grPaint;
-        SkPaint2GrPaintShader(fContext, fRenderTarget, runPaint, viewMatrix, true, &grPaint);
+        if (!SkPaint2GrPaint(fContext, fRenderTarget, runPaint, viewMatrix, true, &grPaint)) {
+            return;
+        }
 
         switch (it.positioning()) {
         case SkTextBlob::kDefault_Positioning: