Make UMA_HISTOGRAM_ENUMERATION work with scoped enums.

ios/chrome/browser/desktop_promotion/desktop_promotion_sync_observer.cc

Index: ios/chrome/browser/desktop_promotion/desktop_promotion_sync_observer.cc
diff --git a/ios/chrome/browser/desktop_promotion/desktop_promotion_sync_observer.cc b/ios/chrome/browser/desktop_promotion/desktop_promotion_sync_observer.cc
index 1b57dacf31eb8d93589ee780291e69b3f4b5c728..067ac418a628ae3c89e4309e1f48a254dfa598a3 100644
--- a/ios/chrome/browser/desktop_promotion/desktop_promotion_sync_observer.cc
+++ b/ios/chrome/browser/desktop_promotion/desktop_promotion_sync_observer.cc
@@ -76,8 +76,8 @@ void DesktopPromotionSyncObserver::OnStateChanged(syncer::SyncService* sync) {
       arraysize(kDesktopIOSPromotionEntrypointHistogramPrefix);
   for (int i = 1; i < entrypoint_prefixes_count + 1; i++) {
     if (sms_entrypoint == i) {

[rkaplow, 2017/03/29 18:02:51]

linear looks like the wrong model here as this is supposed to be:

<enum name="DesktopIOSPromotionEntryPoint" type="int">
  <int value="1" label="Save Passwords new bubble."/>
  <int value="2" label="Bookmarks new bubble."/>
  <int value="3" label="Bookmarks footnote."/>
  <int value="4" label="HistoryPage."/>
</enum>

however I'm not seeing any data for this. maybe it's obsolete anyway? add a todo
to fix or clean it up for an owner?

[dcheng, 2017/03/29 18:24:40]

To elaborate, are you saying that exact linear is the wrong model because the
UMA histogram is defined as an enum?

[Mark P, 2017/03/29 19:03:36]

drive-by: it's not obsolete.  Instead, this feature hasn't been launched yet. 
(The iOS side to the feature is still under review.)

The original enum made sense.  0 is meant to be unused.  These values relate to
sync, and we're using the 0 value in sync to indicate "not set".  No 0 value is
expected to be in the UMA output, which is why when I revised the metrics for
this, I explicitly asked them to leave the 0 out of histograms.xml

[rkaplow, 2017/03/29 23:21:17]

I guess this is ok since it would be strange to have an intermediate enum.
Again, i wouldn't mind a comment explaining why we need LINEAR here.

Unless you think it's worth have an enum for this subset of the enum, it just
seems a bit overkill to me.

[Mark P, 2017/03/29 23:36:33]

That would be overkill, especially because it would require keeping this enum in
sync between chromium code and server-side code.  Right now it only lives on
server-side code.

[dcheng, 2017/03/30 00:41:58]

I added a comment... hopefully it makes sense.

-      UMA_HISTOGRAM_ENUMERATION("DesktopIOSPromotion.SMSSent.IOSSigninReason",
-                                i, entrypoint_prefixes_count + 1);
+      UMA_HISTOGRAM_EXACT_LINEAR("DesktopIOSPromotion.SMSSent.IOSSigninReason",
+                                 i, entrypoint_prefixes_count + 1);
       // If the time delta is negative due to client bad clock we log 0 instead.
       base::Histogram::FactoryGet(
           base::StringPrintf(
@@ -89,8 +89,8 @@ void DesktopPromotionSyncObserver::OnStateChanged(syncer::SyncService* sync) {
       // If the user saw this promotion type, log that it could be a reason
       // for the signin.
       if ((1 << i) & shown_entrypoints)
-        UMA_HISTOGRAM_ENUMERATION("DesktopIOSPromotion.NoSMS.IOSSigninReason",
-                                  i, entrypoint_prefixes_count + 1);
+        UMA_HISTOGRAM_EXACT_LINEAR("DesktopIOSPromotion.NoSMS.IOSSigninReason",
+                                   i, entrypoint_prefixes_count + 1);
     }
   }