Revert of [PageLoadMetrics] Enable AdsPageLoadMetrics by default (patchset #1...

Revert of [PageLoadMetrics] Enable AdsPageLoadMetrics by default (patchset #1 id:1 of https://codereview.chromium.org/2887773003/ )

Reason for revert:
Reverting based on findit-for-me notification

Original issue's description:
> [PageLoadMetrics] Enable AdsPageLoadMetrics by default
>
> Now that we've addressed all of the known causes of crashes, it's time to
> reenable.
>
> BUG=708570
>
> Review-Url: https://codereview.chromium.org/2887773003
> Cr-Commit-Position: refs/heads/master@{#472248}
> Committed: https://chromium.googlesource.com/chromium/src/+/6c08596b40d0fb9e1d66ca96b65b2d1a707fcb26

TBR=jkarlin@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=708570

Review-Url: https://codereview.chromium.org/2890603005
Cr-Commit-Position: refs/heads/master@{#472285}

chrome/browser/page_load_metrics/observers/ads_page_load_metrics_observer.cc

@@ -19,7 +19,8 @@
 
 namespace {
 
-const base::Feature kAdsFeature{"AdsMetrics", base::FEATURE_ENABLED_BY_DEFAULT};
+const base::Feature kAdsFeature{"AdsMetrics",
+                                base::FEATURE_DISABLED_BY_DEFAULT};
 
 bool FrameIsAd(content::NavigationHandle* navigation_handle) {
   // Because sub-resource filtering isn't always enabled, and doesn't work