Add a missing entry to a histogram enum.

I missed this in r486111

BUG=736375

Change-Id: I26b4381945063f7216bab29412bfe0df1c3d95a9
Reviewed-on: https://chromium-review.googlesource.com/575393
Commit-Queue: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487321}

tools/metrics/histograms/histograms.xml

@@ -92764,6 +92764,7 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
       label="in-document existing page renderer-initiated navigation"/>
   <suffix name="ExistingPageDifferentDocumentRendererInitiated"
       label="existing page renderer-initiated navigation"/>
+  <suffix name="SamePage" label="same page navigation"/>
   <suffix name="NewSubFrame" label="new subframe navigation"/>
   <affected-histogram name="Navigation.SecureSchemeHasSSLStatus"/>
 </histogram_suffixes>