Don't call purgeCache() from within GrResource memebers.

R=robertphillips@google.com

Review URL: https://codereview.chromium.org/14864020

git-svn-id: http://skia.googlecode.com/svn/trunk@9102 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/src/gpu/GrRenderTarget.cpp b/src/gpu/GrRenderTarget.cpp
index 5630d58..1cbd5f5 100644
--- a/src/gpu/GrRenderTarget.cpp
+++ b/src/gpu/GrRenderTarget.cpp
@@ -95,28 +95,7 @@
 }
 
 void GrRenderTarget::setStencilBuffer(GrStencilBuffer* stencilBuffer) {
-    if (stencilBuffer == fStencilBuffer) {
-        return;
-    }
-
-    if (NULL != fStencilBuffer) {
-        fStencilBuffer->unref();
-
-        GrContext* context = this->getContext();
-        if (NULL != context) {
-            context->purgeCache();
-        }
-
-        if (NULL != context) {
-            context->purgeCache();
-        }
-    }
-
-    fStencilBuffer = stencilBuffer;
-
-    if (NULL != fStencilBuffer) {
-        fStencilBuffer->ref();
-    }
+    SkRefCnt_SafeAssign(fStencilBuffer, stencilBuffer);
 }
 
 void GrRenderTarget::onRelease() {