Restrict StopLoadingInBackground flag to Android.

The flag was added in cl/734165, but did not restrict to Android,
we want to run the intervention only on Android.

Bug: 763573
Change-Id: I180c32992ef4a36cec616e33d59b418305500265
Reviewed-on: https://chromium-review.googlesource.com/780691Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Shubhie Panicker <panicker@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518422}

chrome/browser/about_flags.cc

@@ -3575,7 +3575,7 @@ const FeatureEntry kFeatureEntries[] = {
 
     {"stop-loading-in-background",
      flag_descriptions::kStopLoadingInBackgroundName,
-     flag_descriptions::kStopLoadingInBackgroundDescription, kOsAll,
+     flag_descriptions::kStopLoadingInBackgroundDescription, kOsAndroid,
      FEATURE_VALUE_TYPE(features::kStopLoadingInBackground)},
 
 #if defined(TOOLKIT_VIEWS)