[AdTagging] Don't track ad scripts that we could detect by context

Ad scripts loaded in ad contexts can be detected by the context
alone. Reserve tracking individual scripts to non-ad contexts.

Bug: 894080

Change-Id: I90ec62e7e816fc83540f0daa232b105d03eb1553
Reviewed-on: https://chromium-review.googlesource.com/c/1276785
Commit-Queue: Josh Karlin <jkarlin@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600549}

third_party/blink/renderer/core/frame/ad_tracker.cc

@@ -114,8 +114,12 @@ void AdTracker::WillSendRequest(ExecutionContext* execution_context,
   if (!request.IsAdResource() && IsAdScriptInStack())
     request.SetIsAdResource();
 
-  // If it is a script marked as an ad, append it to the known ad scripts set.
-  if (resource_type == ResourceType::kScript && request.IsAdResource()) {
+  // If it is a script marked as an ad and it's not in an ad context, append it
+  // to the known ad script set. We don't need to keep track of ad scripts in ad
+  // contexts, because any script executed inside an ad context is considered an
+  // ad script by IsKnownAdScript.
+  if (resource_type == ResourceType::kScript && request.IsAdResource() &&
+      !IsKnownAdExecutionContext(execution_context)) {
     AppendToKnownAdScripts(*execution_context, request.Url().GetString());
   }
 }
@@ -151,9 +155,6 @@ bool AdTracker::IsKnownAdScript(ExecutionContext* execution_context,
   if (!execution_context)
     return false;
 
-  // TODO(jkarlin): Minor memory optimization, stop tracking known ad scripts in
-  // ad contexts. This will reduce the size of known_ad_scripts_. Note that
-  // this is a minor win, as the strings are already ref-counted.
   if (IsKnownAdExecutionContext(execution_context))
     return true;
 

third_party/blink/renderer/core/frame/ad_tracker.h

@@ -85,6 +85,7 @@ class CORE_EXPORT AdTracker : public GarbageCollectedFinalized<AdTracker> {
   // an ad script. Each time the script or function finishes, it pops the stack.
   Vector<bool> stack_frame_is_ad_;
 
+  // The set of ad scripts detected outside of ad-frame contexts.
   HeapHashMap<WeakMember<ExecutionContext>, HashSet<String>> known_ad_scripts_;
 
   DISALLOW_COPY_AND_ASSIGN(AdTracker);