Add assert that memory pool is empty when GrTextBlobCache frees

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1555993005

Review URL: https://codereview.chromium.org/1555993005

src/gpu/GrDrawingManager.cpp

@@ -20,6 +20,9 @@ void GrDrawingManager::cleanup() {
         fDrawTargets[i]->makeClosed();  // no drawTarget should receive a new command after this
         fDrawTargets[i]->clearRT();
 
+        // We shouldn't need to do this, but it turns out some clients still hold onto drawtargets
+        // after a cleanup
+        fDrawTargets[i]->reset();
         fDrawTargets[i]->unref();
     }
 

src/gpu/text/GrTextBlobCache.cpp

@@ -55,4 +55,7 @@ void GrTextBlobCache::freeAll() {
         ++iter;
     }
     fCache.rewind();
+
+    // There should be no allocations in the memory pool at this point
+    SkASSERT(fPool.isEmpty());
 }