Expose gallery watching functionality to the public mediaGalleries API.

BUG=144491

Review URL: https://codereview.chromium.org/440813002

Cr-Commit-Position: refs/heads/master@{#292271}
diff --git a/extensions/browser/extension_function_histogram_value.h b/extensions/browser/extension_function_histogram_value.h
index 35d9887e..0c69033 100644
--- a/extensions/browser/extension_function_histogram_value.h
+++ b/extensions/browser/extension_function_histogram_value.h
@@ -943,6 +943,10 @@
   EASYUNLOCKPRIVATE_GETREMOTEDEVICES,
   FILESYSTEMPROVIDER_GETALL,
   EASYUNLOCKPRIVATE_CONNECTTOBLUETOOTHSERVICEINSECURELY,
+  MEDIAGALLERIES_ADDGALLERYWATCH,
+  MEDIAGALLERIES_REMOVEGALLERYWATCH,
+  MEDIAGALLERIES_GETALLGALLERYWATCH,
+  MEDIAGALLERIES_REMOVEALLGALLERYWATCH,
   // Last entry: Add new entries above and ensure to update
   // tools/metrics/histograms/histograms.xml.
   ENUM_BOUNDARY