Remove UMA metric Event.Scroll.ScrollerSize.OnScroll

We no longer need the metric since the experiment of not compositing
small scrollers is done. See crbug.com/684631.

Bug: 775643
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_chromium_dbg_ng
Change-Id: I689f26f1355f01238f3ac8f0121bca59e9be3956
Reviewed-on: https://chromium-review.googlesource.com/728702
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Commit-Queue: Yi Gu <yigu@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#510861}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: c3c92b5963257e19dbc255ed9426d42160b051f2

check_gn_headers_whitelist.txt

@@ -264,7 +264,6 @@ third_party/WebKit/Source/platform/graphics/cpu/arm/WebGLImageConversionNEON.h
 third_party/WebKit/Source/platform/graphics/cpu/mips/WebGLImageConversionMSA.h
 third_party/WebKit/Source/platform/graphics/paint/PaintImage.h
 third_party/WebKit/Source/platform/scheduler/base/task_queue.h
-third_party/WebKit/Source/platform/scroll/ScrollerSizeMetrics.h
 third_party/WebKit/Source/platform/text/TabSize.h
 third_party/WebKit/Source/platform/text/TextDirection.h
 third_party/WebKit/Source/platform/transforms/TransformOperation.h