[Metrics] Add entry to V8CacheBehaviour enum.

Adds kHitIsolateCacheWhenStreamingSource.

BUG=chromium:865098
TBR=delphick@chromium.org

Change-Id: I9112410a1ddf9b9f56667a7880152d306d4a2612
Reviewed-on: https://chromium-review.googlesource.com/1163672
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580883}

tools/metrics/histograms/enums.xml

@@ -48309,6 +48309,10 @@ Full version information for the fingerprint enum values:
     Blink did not request a code cache because it has a script resource without
     a cache handler
   </int>
+  <int value="20" label="kHitIsolateCacheWhenStreamingSource">
+    Blink parsed and compiled the script as it was streamed, but we then hit
+    V8's isolate's compilation cache and did not require the streamed result.
+  </int>
 </enum>
 
 <enum name="V8CodeCacheRejectReason">