Adds Server Previews protocol UMA plus UMA for not accepting previews

tools/metrics/histograms/enums.xml

Index: tools/metrics/histograms/enums.xml
diff --git a/tools/metrics/histograms/enums.xml b/tools/metrics/histograms/enums.xml
index fda1ce096b0d23fe10823453f76d7f27f851b9e5..5f2d3d8db9ce8c82cd2aa69911dcce5cd7b1e03c 100644
--- a/tools/metrics/histograms/enums.xml
+++ b/tools/metrics/histograms/enums.xml
@@ -7053,6 +7053,21 @@ uploading your change for review.  These are checked by presubmit scripts.
   <int value="3" label="Enabled from second screen"/>
 </enum>
 
+<enum name="DataReductionProxyProtocolAcceptTransformEvent">
+  <int value="0" label="AcceptTransform lite-page sent"/>
+  <int value="1" label="ContentTransform lite-page received"/>
+  <int value="2" label="Page policy for empty-image received"/>
+  <int value="3" label="AcceptTransform empty-image sent"/>
+  <int value="4" label="ContentTransform empty-image received"/>
+  <int value="5" label="AcceptTransform compressed-video sent"/>

[bengr, 2017/06/26 18:11:47]

Is there a corresponding CT value for this one?

[dougarnett, 2017/06/26 21:19:16]

No, Flywheel will respond with a 302 redirect to the compressed resource
(according to the g3doc). Not sure I want to expand the scope for that (rather
skip including the cpat for compressed video instead for this iteration).

+</enum>
+
+<enum name="DataReductionProxyProtocolNotAcceptingTransformReason">
+  <int value="0" label="Not accepting proxy transforms, transforms disabled"/>

[bengr, 2017/06/26 18:11:48]

Disabled by what?

[dougarnett, 2017/06/26 21:19:16]

Eventually, either by finch or user flag disabling the feature.

Currently though (depending on blacklist fieldtrial), it may be disabled for one
of 3 cases (flag, too many opt outs, too many loads of placeholders). There is
currently no reason plumbed through SetLoFiModeOff() to let us distinguish. I
think the latter 2 go away with the new blacklist support (which does have a
separate code).

Clarified that this is disabled on the client though.

+  <int value="1" label="Not accepting proxy transforms, page blacklisted"/>

[bengr, 2017/06/26 18:11:48]

By client side blacklist?

[dougarnett, 2017/06/26 21:19:16]

Done.

+  <int value="2" label="Not accepting proxy transforms, cellular-only mode"/>

[bengr, 2017/06/26 18:11:48]

And the network is not cellular? Be explicit.

[dougarnett, 2017/06/26 21:19:16]

Done.

+</enum>
+
 <enum name="DataReductionProxyProxyPrefMigrationResult">
   <int value="0" label="Proxy pref not cleared"/>
   <int value="1" label="Empty proxy pref was cleared"/>