[SH] Update shared_highlighting histogram enums to fit guidelines.

Added explicit values to enum entries and added a comment mentioning to
not change or re-use those values.

Also made histogram definition clearer.

Change-Id: Idada23970bd834575d314bcfac941f5a3b0e255c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2480582
Auto-Submit: sebsg <sebsg@chromium.org>
Commit-Queue: Mark Pearson <mpearson@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818019}

components/shared_highlighting/core/common/shared_highlighting_metrics.h

@@ -9,26 +9,31 @@
 
 namespace shared_highlighting {
 
-// Update corresponding |LinkGenerationError| in enums.xml.
+// These values are persisted to logs. Entries should not be renumbered and
+// numeric values should never be reused.
+// The type of errors that can happen during link generation.
 enum class LinkGenerationError {
-  kIncorrectSelector,
-  kNoRange,
-  kNoContext,
-  kContextExhausted,
-  kContextLimitReached,
-  kEmptySelection,
-
-  kTabHidden,
-  kOmniboxNavigation,
-  kTabCrash,
+  kIncorrectSelector = 0,
+  kNoRange = 1,
+  kNoContext = 2,
+  kContextExhausted = 3,
+  kContextLimitReached = 4,
+  kEmptySelection = 5,
+
+  // Android specific.
+  kTabHidden = 5,
+  kOmniboxNavigation = 6,
+  kTabCrash = 7,
 
   kMaxValue = kTabCrash
 };
 
-// Update corresponding |TextFragmentLinkOpenSource| in enums.xml.
+// These values are persisted to logs. Entries should not be renumbered and
+// numeric values should never be reused.
+// The different sources from which a text fragment URL can come from.
 enum class TextFragmentLinkOpenSource {
-  kUnknown,
-  kSearchEngine,
+  kUnknown = 0,
+  kSearchEngine = 1,
 
   kMaxValue = kSearchEngine,
 };

tools/metrics/histograms/histograms_xml/others/histograms.xml

@@ -14538,14 +14538,14 @@ should be kept until we remove incident reporting. -->
     (e.g. #:=:text=SELECTOR). Indicates source type that navigation originated
     from. It only know about search engines that come from the pre-populated
     list installed with Chrome. If the user uses a search engine not on the
-    list, it won't count as a search engine. This is even true if the user have
-    a custom search engine. These custom search engines can be created by hand
-    or auto-generated. In either case, it's not going to count as a search
-    engine. Also, it only checks to see if the *referrer* is from a domain
-    associated with search engine. If someone posts a link somewhere on a web
-    site that happens to have a search engine known to Google and that links
-    includes the fragment tokens, then if a user clicks that link, it'll look
-    like it came from a search engine even thought they did not.
+    list, it will be recorded as Unknown. This is even true if the user have a
+    custom search engine. These custom search engines can be created by hand or
+    auto-generated. In either case, it's not going to count as a search engine.
+    Also, it only checks to see if the *referrer* is from a domain associated
+    with search engine. If someone posts a link somewhere on a web site that
+    happens to have a search engine known to Google and that links includes the
+    fragment tokens, then if a user clicks that link, it'll look like it came
+    from a search engine even thought they did not.
   </summary>
 </histogram>