Fix typo in a histogram name.

R=asvitkine@chromium.org

Review URL: https://codereview.chromium.org/875983007

Cr-Commit-Position: refs/heads/master@{#314331}

tools/metrics/histograms/histograms.xml

@@ -39942,7 +39942,7 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   <summary>Time spent in V8 compiler (full codegen).</summary>
 </histogram>
 
-<histogram name="V8.CompileCacheableMicroseconds" units="microseconds">
+<histogram name="V8.CompileCacheableMicroSeconds" units="microseconds">
   <owner>yangguo@chromium.org</owner>
   <summary>
     Time spent compiling a script that may be subject to caching.