Reject unadvertised encodings

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index 21f1ae8975fb61db1c74a2a900910d8e28b7c134..831340f07eb5f00716e289824781d1ec2bcaf91b 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -102608,6 +102608,7 @@ from previous Chrome versions.
   <int value="5" label="SDCHPossible"/>
   <int value="6" label="Invalid"/>
   <int value="7" label="None"/>
+  <int value="8" label="Unknown"/>

[Randy Smith (Not in Mondays), 2017/03/21 21:23:20]

Given that we need to only reject known encodings that aren't in Accept-Encoding
(as per https://bugs.chromium.org/p/chromium/issues/detail?id=579606#c26 and
https://codereview.chromium.org/2753453003#msg11) let's add yet another entry
here, which is "rejected", meaning "Yeah, we've heard of it, but we didn't tell
the server it could use that encoding, so we aren't going to handle it".

Also, I want to call out that there's are two ways in which I'm being less than
pure about this histogram (because I just don't think doing it precisely right
is worth the hassle).  The first is that because we're recording in this
histogram from both below and above the cache, comparisons between the
"Rejected" bucket and other buckets will be apples and close-to-but-not-quite
apples; specifically, entries that make it into the cache will have many chances
to be recorded in the histogram, as opposed to just one for the entries that are
rejected below the cache.  

The second is that in some sense modifying the histogram should mean revving it
(i.e. moving to "ContentDecodingFailed3"), but we're not modifying the meaning
of any of the existing buckets, just adding new ones, so if the people
evaluating the histogram remember that it shouldn't be a problem.  And this
histogram isn't examined that frequently anyway, so within a couple of releases
it also shouldn't be a problem.

[eustas, 2017/03/27 10:27:11]

I'm fine with either solution. IMHO having less histograms is better - easier to
see the picture as a whole.

[Randy Smith (Not in Mondays), 2017/03/28 19:55:36]

Sorry, I wasn't proposing two solutions, I was asking you to change "Unknown" to
"Rejected" and add a separate "Unknown" which would be reported above the cache.
 The rest of the comments were documentation for other reviewers to challenge me
on being imprecise, and documentation for future code archaeologists.

 </enum>
 
 <enum name="NetInternalsUiFeature" type="int">