fix missing clipmaskmanager

Review URL: https://codereview.chromium.org/1330213004 .
diff --git a/src/gpu/GrDrawTarget.cpp b/src/gpu/GrDrawTarget.cpp
index 6c9c386..076b626 100644
--- a/src/gpu/GrDrawTarget.cpp
+++ b/src/gpu/GrDrawTarget.cpp
@@ -39,6 +39,7 @@
     , fLastFlushToken(0) {
     // TODO: Stop extracting the context (currently needed by GrClipMaskManager)
     fContext = fGpu->getContext();
+    fClipMaskManager.reset(new GrClipMaskManager(this));
 }
 
 GrDrawTarget::~GrDrawTarget() {