[v8][wasm] Add two more histograms for code GC

This adds two more histograms to measure the time it takes per GC, and
the number of GCs executed per native module.
Both expire end of March 2020, when also the other wasm histograms
expire, so we can review them all together for extension.
The histograms were added to v8 in https://crrev.com/c/1615246 and
https://crrev.com/c/1615250.

R=mpearson@chromium.org
CC=​mstarzinger@chromium.org

Bug: v8:8217, chromium:956480
Change-Id: Ie316b53330f960df2c5fac3e38b67ac59b7e80d3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1615203
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662126}

tools/metrics/histograms/histograms.xml

@@ -136272,6 +136272,19 @@ should be kept until we use this API.  -->
   </summary>
 </histogram>
 
+<histogram name="V8.WasmCodeGCTime" units="microseconds"
+    expires_after="2020-03-31">
+  <owner>titzer@chromium.org</owner>
+  <owner>adamk@chromium.org</owner>
+  <owner>clemensh@chromium.org</owner>
+  <summary>
+    Time to execute a single WebAssembly code GC, measured from when it is
+    triggered until all isolates reported live code and all dead code was freed.
+    Recorded after each WebAssembly code GC if a high-resolution clock is
+    available.
+  </summary>
+</histogram>
+
 <histogram name="V8.WasmCompileFunctionMicroSeconds" units="microseconds">
   <owner>titzer@chromium.org</owner>
   <owner>adamk@chromium.org</owner>
@@ -136528,6 +136541,17 @@ should be kept until we use this API.  -->
   </summary>
 </histogram>
 
+<histogram name="V8.WasmModuleNumberOfCodeGCsTriggered" units="gcs"
+    expires_after="2020-03-31">
+  <owner>titzer@chromium.org</owner>
+  <owner>adamk@chromium.org</owner>
+  <owner>clemensh@chromium.org</owner>
+  <summary>
+    Number of code GCs triggered per WebAssembly module. Recorded after each
+    code GC.
+  </summary>
+</histogram>
+
 <histogram name="V8.WasmModuleSizeBytes" units="bytes">
   <owner>titzer@chromium.org</owner>
   <owner>adamk@chromium.org</owner>