Add a histogram (Net.PacResultForStrippedUrl) that estimates how often PAC scripts depend on the UR…

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 60140b3367e41a614910052a1d7bdff63781a7fb..9057bb98e6168836735be94189222f50cfa1396a 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -26072,6 +26072,17 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Net.PacResultForStrippedUrl" enum="PacResultForStrippedUrl">
+  <owner>eroman@chromium.org</owner>
+  <summary>
+    Estimates the impact of PAC script results when the URL path and parameters

[Mark P, 2016/03/18 21:34:50]

Histogram descriptions should clearly explain when the histogram is emitted.
Also, "Estimates" is an inappropriate word.  The histogram doesn't estimate
anything.  It's an exact count of something.
Also, "impact" is so vague that I have no idea what it means.

[eroman, 2016/03/18 23:15:16]

Thanks for the feedback.
Here is my new and improved wording:

    Proxy Auto Config (PAC) allows specifying an arbitrary javascript program
    to pick network proxies on a per-request basis (based on the URL). This
    works by calling the script's FindProxyForURL() function with the
    target URL as its first argument.

    Traditionally, the URL parameter passed into the script contains the exact
    URL being loaded, except maybe for having stripped the fragment and
    embedded identity portions of the URL.

    This histogram records what happens when the URL passed into
    FindProxyForURL() additionally has had its path component stripped. Does it
    return the same proxy list as when calling FindProxyForURL() with the
    original (unmodified) URL?

    This comparison is done only when the URL scheme implies a secure channel
    (i.e. https:// and wss://), in order to gather data for crbug.com/593759.

+    are elided, by running the PAC script a second time with a fixed path.
+
+    This is only measured for https:// and wss:// URL schemes in order to gather
+    data for crbug.com/593759.
+  </summary>
+</histogram>
+
 <histogram name="Net.Ping_ResponseStartedTime" units="ms">
   <obsolete>
     Deprecated 4/16/2014. No longer tracked.
@@ -76131,6 +76142,21 @@ To add a new entry, add it with any value and run test to compute valid value.
   <int value="4" label="Index"/>
 </enum>
 
+<enum name="PacResultForStrippedUrl" type="int">
+  <summary>
+    Result for PAC script experiment as defined in
+    net/proxy/proxy_resolver_v8_tracing.h
+  </summary>
+  <int value="0" label="SKIPPED_FALLBACK_BLOCKING_DNS"/>
+  <int value="1" label="SUCCESS"/>
+  <int value="2" label="SUCCESS_DIFFERENT_ALERTS"/>
+  <int value="3" label="SUCCESS_DIFFERENT_NUM_DNS"/>
+  <int value="4" label="FAIL_ABANDONED"/>
+  <int value="5" label="FAIL_FALLBACK_BLOCKING_DNS"/>
+  <int value="6" label="FAIL_ERROR"/>
+  <int value="7" label="FAIL_DIFFERENT_PROXY_LIST"/>
+</enum>
+
 <enum name="PageLoadEvent" type="int">
   <obsolete>
     Deprecated in favor of {Committed|Provisional|InternalError}LoadEvent