Enable AsyncStackAdTagging by default

To be cautious we landed this patch behind a feature to make sure it
wasn't hurting performance. We're not seeing anything in 28 day data
on canary/dev to suggest that it's problematic, so launching via
waterfall.

Bug: 851531
Change-Id: I77fc899170d8b5f76e7d50a4db3a4e33e88b40ac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2053411Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Josh Karlin <jkarlin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#743597}

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

@@ -37,7 +37,7 @@ namespace features {
 // Controls whether the AdTracker will look across async stacks to determine if
 // the currently running stack is ad related.
 const base::Feature kAsyncStackAdTagging{"AsyncStackAdTagging",
-                                         base::FEATURE_DISABLED_BY_DEFAULT};
+                                         base::FEATURE_ENABLED_BY_DEFAULT};
 
 // Controls whether the AdTracker analyzes the whole pseudo-stack or just the
 // top of the stack when detecting ads.