Add Wasm code space metric to histograms.xml

The metric was added to V8 in https://crrev.com/c/1061497 to measure
the code size increase of Liftoff and get data to find a good upper
limit on the wasm code space.

R=mpearson@chromium.org
CC=titzer@chromium.org

Bug: chromium:787421
Change-Id: I6aee5a5633051be376f81b4e9fffbf036bc44ec1
Reviewed-on: https://chromium-review.googlesource.com/1064238Reviewed-by: Mark Pearson <mpearson@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#560178}

tools/metrics/histograms/histograms.xml

@@ -101375,6 +101375,15 @@ uploading your change for review.
   </summary>
 </histogram>
 
+<histogram name="V8.WasmModuleCodeSizeMiB" units="MB">
+  <owner>titzer@chromium.org</owner>
+  <owner>clemensh@chromium.org</owner>
+  <summary>
+    The amount of code space in MiB used by individual WebAssembly modules.
+    Recorded for each module on finalization, i.e. when it is garbage-collected.
+  </summary>
+</histogram>
+
 <histogram name="V8.WasmModuleSizeBytes" units="bytes">
   <owner>bradnelson@chromium.org</owner>
   <owner>titzer@chromium.org</owner>