Add cookie prefix metrics

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 4132996a123981b68b3d1702cf59dd2010eaff41..71fb78409490b3133ac988dc22389836a6562ce1 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -5229,6 +5229,23 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <summary>Intervals between access time updates for each cookie.</summary>
 </histogram>
 
+<histogram name="Cookie.CookiePrefix" enum="CookiePrefix">
+  <owner>estark@chromium.org</owner>
+  <summary>
+    Records cookies with prefixes used by the cookie prefix proposal in
+    https://tools.ietf.org/html/draft-west-cookie-prefixes.

[Mark P, 2015/11/20 17:58:53]

All histograms should clearly say when they are emitted.

[estark, 2015/11/20 18:19:32]

Done, PTAL.

[Mark P, 2015/11/20 19:01:42]

Not done in the latest patchset.

For example, is this recorded every time a cookie is read?  Every time the
cookie is updated?  The first time a cookie is created?  When a cookie is sent
to a server?  Once per pageload, regardless of how many times the cookie was
updated on the pageload?  etc.

ditto in the histogram below

[Mark P, 2015/11/20 19:01:42]

Not done in the latest patchset.

You need to say when the histogram is emitted.  For instance, is it emitted
every time the cookie is read?  Every time it is updated?  When it is first
created?  Every time it is sent to a server?  Once per pageload in which the
cookie is touched, regardless of how many times the cookie is touched on that
pageload?  etc.

ditto in the histogram below

[estark, 2015/11/20 19:18:27]

Sorry but I'm having trouble understanding what isn't clear in what I wrote in
patch set #5: "Number of times a cookie is set..." To me that seems to describe
exactly when the histogram is emitted: whenever we set a cookie with the
described properties, we emit the histogram. Would either of the following
options be more clear to you?

- "Number of times a cookie is set in the cookie store..."
- "Number of times a cookie is set in the cookie store via a Set-Cookie header
or setting document.cookie..."

[Mark P, 2015/11/20 20:13:43]

Oops, somehow I missed the "is set" section.

+  </summary>
+</histogram>
+
+<histogram name="Cookie.CookiePrefixBlocked" enum="CookiePrefix">
+  <owner>estark@chromium.org</owner>
+  <summary>
+    Records cookies with prefixes used by the cookie prefix proposal in
+    https://tools.ietf.org/html/draft-west-cookie-prefixes that would be blocked
+    under that proposal.
+  </summary>
+</histogram>
+
 <histogram name="Cookie.CookieSourceScheme" enum="CookieSourceScheme">
   <owner>estark@chromium.org</owner>
   <summary>
@@ -56511,6 +56528,12 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="3" label="Both"/>
 </enum>
 
+<enum name="CookiePrefix" type="int">
+  <int value="0" label="No special prefix"/>
+  <int value="1" label="Secure prefix"/>
+  <int value="2" label="Host prefix"/>
+</enum>
+
 <enum name="CookieSourceScheme" type="int">
   <int value="0" label="Secure cookie, source scheme is cryptographic"/>
   <int value="1" label="Secure cookie, source scheme is not cryptographic"/>