Count getComputedStyle() for -webkit-appearance

Now getComputedStyle() for -webkit-appearance returns adjusted values.
See [1].  We'd like to know whether we can remove this behavior or not.

[1] whatwg/compat#6 (comment)

Bug: 810162
Change-Id: Icc17d681d7105d8a77ff07e18b7f270bd6918f01
Reviewed-on: https://chromium-review.googlesource.com/936523
Reviewed-by: Takayoshi Kochi <kochi@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#539142}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 73c0ec5abceb2d983cbdbf5f3e95e3d1371822df

metrics/histograms/enums.xml

@@ -17871,6 +17871,7 @@ Called by update_net_error_codes.py.-->
   <int value="2371" label="AsyncClipboardAPIReadText"/>
   <int value="2372" label="AsyncClipboardAPIWriteText"/>
   <int value="2373" label="OpenerNavigationWithoutGesture"/>
+  <int value="2374" label="GetComputedStyleWebkitAppearance"/>
 </enum>
 
 <enum name="FeedbackSource">