Clean up syncer::StopSource enum

- Deprecate the PROFILE_DESTRUCTION entry which was unused.
- Add explicit numeric values to the remaining entries (as is
  recommended for enums used in UMA histograms).

Bug: 939752
Change-Id: Iedd144b5a958a4bd55c78b1e25fba23d8d9f2ed4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2467956
Auto-Submit: Marc Treib <treib@chromium.org>
Reviewed-by: Maksim Moskvitin <mmoskvitin@google.com>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816578}

components/sync/base/stop_source.h

@@ -13,13 +13,13 @@ namespace syncer {
 // A Java counterpart will be generated for this enum.
 // GENERATED_JAVA_ENUM_PACKAGE: org.chromium.components.sync
 enum StopSource {
-  PROFILE_DESTRUCTION,   // The user destroyed the profile.
-  SIGN_OUT,              // The user signed out of Chrome.
-  BIRTHDAY_ERROR,        // A dashboard stop-and-clear on the server.
-  CHROME_SYNC_SETTINGS,  // The on/off switch in settings for mobile Chrome.
-  ANDROID_CHROME_SYNC,   // Android's sync setting for Chrome.
-  ANDROID_MASTER_SYNC,   // Android's master sync setting.
-  STOP_SOURCE_LIMIT,
+  // Deprecated: PROFILE_DESTRUCTION = 0,
+  SIGN_OUT = 1,              // The user signed out of Chrome.
+  BIRTHDAY_ERROR = 2,        // A dashboard stop-and-clear on the server.
+  CHROME_SYNC_SETTINGS = 3,  // The on/off switch in settings for mobile Chrome.
+  ANDROID_CHROME_SYNC = 4,   // Android's sync setting for Chrome.
+  ANDROID_MASTER_SYNC = 5,   // Android's master sync setting.
+  STOP_SOURCE_LIMIT = 6,
 };
 
 }  // namespace syncer