Add unit tests to text context

TBR=bsalomon@google.com
BUG=skia:

Review URL: https://codereview.chromium.org/1128153005
diff --git a/src/gpu/GrTextContext.cpp b/src/gpu/GrTextContext.cpp
index 6153f3b..4c6dabc 100644
--- a/src/gpu/GrTextContext.cpp
+++ b/src/gpu/GrTextContext.cpp
@@ -66,6 +66,7 @@
     } while (textContext);
 
     // fall back to drawing as a path
+    SkASSERT(fGpuDevice);
     this->drawTextAsPath(skPaint, viewMatrix, text, byteLength, x, y, clipBounds);
 }
 
@@ -89,6 +90,7 @@
     } while (textContext);
 
     // fall back to drawing as a path
+    SkASSERT(fGpuDevice);
     this->drawPosTextAsPath(skPaint, viewMatrix, text, byteLength, pos, scalarsPerPosition, offset,
                             clipBounds);
 }