Add MobileWillEnterForeground user action for iOS

Logged when app transitioned from background to foreground.
Used for Breadcrumbs to log steps to reproduce for crashlogs.

Bug: 1046231
Change-Id: I6cc6b7d22fd19ce71347defbfb9246b3788125db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2085598
Commit-Queue: Eugene But <eugenebut@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Auto-Submit: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747366}

ios/chrome/app/application_delegate/app_state.mm

@@ -344,6 +344,8 @@ initWithBrowserLauncher:(id<BrowserLauncher>)browserLauncher
     feature_engagement::TrackerFactory::GetForBrowserState(currentBrowserState)
         ->NotifyEvent(feature_engagement::events::kChromeOpened);
   }
+
+  base::RecordAction(base::UserMetricsAction("MobileWillEnterForeground"));
 }
 
 - (void)resumeSessionWithTabOpener:(id<TabOpening>)tabOpener

tools/metrics/actions/actions.xml

@@ -14191,6 +14191,17 @@ should be able to be added at any place in this file.
   </description>
 </action>
 
+<action name="MobileWillEnterForeground">
+  <owner>eugenebut@chromium.org</owner>
+  <owner>marq@chromium.org</owner>
+  <description>
+    App transitioned from background to foreground. This action is directly or
+    indirectly triggered by the user. Example of direct trigger: the user tapped
+    on app icon or choose the app in app switcher. Example of indirect trigger:
+    the user tapped on GMail link or used Today's View extension. iOS only.
+  </description>
+</action>
+
 <action name="MostVisited0">
   <obsolete>No longer recorded.</obsolete>
   <owner>Please list the metric's owners. Add more owner tags as needed.</owner>