AW UMA: clarify the GetAppPackageName() method

No change to logic, only docs. This clarifies that embedders may return
the empty string from GetAppPackageName() for privacy concerns
(previously this was not documented as a supported case).

This removes embedder-specific terminology from the metrics component,
and clarifies WebView's behavior in its method override.

Bug: 969803
Test: N/A
Change-Id: Idb57e4de11cc8c40b58f95a012a5110ba9bdf8b0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1897012Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712688}

android_webview/browser/metrics/aw_metrics_service_client.h

@@ -127,6 +127,9 @@ class AwMetricsServiceClient : public metrics::MetricsServiceClient,
       override;
   base::TimeDelta GetStandardUploadInterval() override;
   bool ShouldStartUpFastForTesting() const override;
+
+  // Gets the embedding app's package name if it's OK to log. Otherwise, this
+  // returns the empty string.
   std::string GetAppPackageName() override;
 
  protected:

components/metrics/metrics_service_client.h

@@ -142,8 +142,11 @@ class MetricsServiceClient {
   // Returns whether UKM notification listeners were attached to all profiles.
   virtual bool AreNotificationListenersEnabledOnAllProfiles();
 
-  // Gets Chrome's package name in Android Chrome, or the host app's package
-  // name in Android WebView, or an empty string on other platforms.
+  // Gets the app package name (as defined by the embedder). Since package name
+  // is only meaningful for Android, other platforms should return the empty
+  // string (this is the same as the default behavior). If the package name
+  // should not be logged for privacy/fingerprintability reasons, the embedder
+  // should return the empty string.
   virtual std::string GetAppPackageName();
 
   // Gets the key used to sign metrics uploads. This will be used to compute an