Initialize wrapped render target wrap status correctly

Make GrGLGpu::onWrapBackendRenderTarget update "is wrapped" flag.
Otherwise accessing the flag results in uninitialized read, possibly
resulting in wrapped render target ending up in the cache.

Review URL: https://codereview.chromium.org/818023002

src/gpu/gl/GrGpuGL.cpp

@@ -418,6 +418,7 @@ GrRenderTarget* GrGLGpu::onWrapBackendRenderTarget(const GrBackendRenderTargetDe
     idDesc.fRTFBOID = static_cast<GrGLuint>(wrapDesc.fRenderTargetHandle);
     idDesc.fMSColorRenderbufferID = 0;
     idDesc.fTexFBOID = GrGLRenderTarget::kUnresolvableFBOID;
+    idDesc.fIsWrapped = true;
 
     GrSurfaceDesc desc;
     desc.fConfig = wrapDesc.fConfig;