Add a use counter of css clip of fixed-pos elements.

BUG=644510

CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2326773002
Cr-Commit-Position: refs/heads/master@{#417524}

third_party/WebKit/Source/core/frame/UseCounter.h

@@ -1285,7 +1285,7 @@ public:
         MetaSetCookieWhenCSPBlocksInlineScript = 1549,
         MetaRefreshWhenCSPBlocksInlineScript = 1550,
         MiddleClickAutoscrollStart = 1551,
-
+        ClipCssOfFixedPositionElement = 1552,
         // Add new features immediately above this line. Don't change assigned
         // numbers of any item, and don't reuse removed slots.
         // Also, run update_use_counter_feature_enum.py in chromium/src/tools/metrics/histograms/

third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp

@@ -501,6 +501,8 @@ PaintLayerPainter::PaintResult PaintLayerPainter::paintLayerWithTransform(Graphi
             if (needsToClip(paintingInfo, clipRectForFragment)) {
                 if (m_paintLayer.layoutObject()->isPositioned() && clipRectForFragment.isClippedByClipCss())
                     UseCounter::count(m_paintLayer.layoutObject()->document(), UseCounter::ClipCssOfPositionedElement);
+                if (m_paintLayer.layoutObject()->isFixedPositioned())
+                    UseCounter::count(m_paintLayer.layoutObject()->document(), UseCounter::ClipCssOfFixedPositionElement);
                 clipRecorder.emplace(context, *parentLayer->layoutObject(), DisplayItem::kClipLayerParent, clipRectForFragment, &paintingInfo, fragment.paginationOffset, paintFlags);
             }
         }

tools/metrics/histograms/histograms.xml

@@ -80681,6 +80681,7 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="1549" label="MetaSetCookieWhenCSPBlocksInlineScript"/>
   <int value="1550" label="MetaRefreshWhenCSPBlocksInlineScript"/>
   <int value="1551" label="MiddleClickAutoscrollStart"/>
+  <int value="1552" label="ClipCssOfFixedPositionElement"/>
 </enum>
 
 <enum name="FetchRequestMode" type="int">