[WeakMemoryCache] Do not prune in didAccessDecodedData()

Preparation for removing didAccessDecodedData()
in https://codereview.chromium.org/2411243004.

BUG=603462

Review-Url: https://codereview.chromium.org/2435873003
Cr-Commit-Position: refs/heads/master@{#427267}

third_party/WebKit/Source/core/fetch/Resource.cpp

@@ -806,7 +806,6 @@ void Resource::setEncodedSizeMemoryUsage(size_t encodedSize) {
 
 void Resource::didAccessDecodedData() {
   memoryCache()->updateDecodedResource(this, UpdateForAccess);
-  memoryCache()->prune();
 }
 
 void Resource::finishPendingClients() {