Add histograms for a number of hacks in the HTTP header parsing logic.

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 244c41c92f71bc787d1bf97d0282fc80d3ce61c4..bed1cf4f1fae56aab00c61943bd8b63b5144d27d 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -18627,6 +18627,15 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="Net.HttpHeaderParserEvent" enum="HttpHeaderParserEvent">
+  <owner>mmenke@chromium.org</owner>
+  <summary>
+    Records the frequency with which a number of hacky HTTP header parsing rules
+    are invoked.  This histogram should be removed after we have enough data to
+    know if we can remove the hacks.
+  </summary>
+</histogram>
+
 <histogram name="net.HttpIdentSrcURL" units="requests">
   <owner>tsepez@chromium.org</owner>
   <summary>
@@ -52496,6 +52505,14 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   <int value="15" label="Negotiate Secure Server"/>
 </enum>
 
+<enum name="HttpHeaderParserEvent" type="int">
+  <int value="0" label="PARSER_INVOKED"/>
+  <int value="1" label="HTTP_09_RESPONSE"/>
+  <int value="2" label="ALLOWED_TRUNCATED_HEADERS"/>
+  <int value="3" label="SKIPPED_WS_PREFIX"/>
+  <int value="4" label="SKIPPED_NON_WS_PREFIX"/>
+</enum>
+
 <enum name="HttpPipelineStatus" type="int">
   <int value="0" label="Success"/>
   <int value="1" label="Redirected"/>