Opt-in non-default writing modes scroll offsets into UseCounter UKM

This feature is counted when a document which has a scroller with
non-default writing mode (horizontal-rl or vertical-rl) is scrolled
with a positive scroll offset.

This counter will be used to reason about the web-compat impact of
shipping spec compliant scroll offsets (gated behind CSSOMViewScrollCoordinates)


1) The collection has been approved by privacy review
2) The feature is rare as measured by UMA (0.0X% of page loads) and
   satisfies <1% of page views requirement for this counter.

For more info see the intent-to-ship thread:
https://groups.google.com/a/chromium.org/g/blink-dev/c/7X2CKPGeEa0/m/PqICcwwMBgAJ

Bug: 721759

Change-Id: I1a49590e5e92657ca8cdf588d2f57991e569cd4b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2055350Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Majid Valipour <majidvp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#742201}

components/page_load_metrics/browser/observers/use_counter/ukm_features.cc

@@ -151,6 +151,8 @@ UseCounterPageLoadMetricsObserver::GetAllowedUkmFeatures() {
           WebFeature::kV8MediaSession_SetActionHandler_Method,
           WebFeature::kLargeStickyAd,
           WebFeature::kOverlayInterstitialAd,
+          WebFeature::
+              kElementWithLeftwardOrUpwardOverflowDirection_ScrollLeftOrTopSetPositive,
       }));
   return *opt_in_features;
 }