[css-flexbox] Add a use counter for single-line align-content

The CSSWG would like to apply align-content to single-line flexboxes
as well:
https://lists.w3.org/Archives/Public/www-style/2018Nov/0007.html
https://github.com/w3c/csswg-drafts/issues/3052

Add a use counter for that to see if it is web-compatible.

TESTED=Manually using chrome://histograms
R=eae@chromium.org, tabatkins@google.com

Change-Id: I3af67474937c8c589e8235c451b358b839556d1f
Reviewed-on: https://chromium-review.googlesource.com/c/1354539
Commit-Queue: Christian Biesinger <cbiesinger@chromium.org>
Commit-Queue: Emil A Eklund <eae@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611984}

third_party/blink/public/platform/web_feature.mojom

@@ -2091,6 +2091,7 @@ enum WebFeature {
   kWebKitUserModifyReadOnlyEffective = 2639,
   kWebKitUserModifyPlainTextEffective = 2640,
   kCSSAtRuleFontFeatureValues = 2641,
+  kFlexboxSingleLineAlignContent = 2642,
 
   // Add new features immediately above this line. Don't change assigned
   // numbers of any item, and don't reuse removed slots.

third_party/blink/renderer/core/layout/layout_flexible_box.cc

@@ -1511,14 +1511,19 @@ void LayoutFlexibleBox::LayoutColumnReverse(FlexItemVectorView& children,
 }
 
 void LayoutFlexibleBox::AlignFlexLines(Vector<FlexLine>& line_contexts) {
-  if (line_contexts.IsEmpty() || !IsMultiline())
-    return;
-
   const StyleContentAlignmentData align_content =
       FlexLayoutAlgorithm::ResolvedAlignContent(StyleRef());
   if (align_content.GetPosition() == ContentPosition::kFlexStart)
     return;
 
+  if (IsMultiline() && !line_contexts.IsEmpty()) {
+    UseCounter::Count(GetDocument(),
+                      WebFeature::kFlexboxSingleLineAlignContent);
+  }
+
+  if (line_contexts.IsEmpty() || !IsMultiline())
+    return;
+
   LayoutUnit available_cross_axis_space = CrossAxisContentExtent();
   for (const FlexLine& line : line_contexts)
     available_cross_axis_space -= line.cross_axis_extent;

tools/metrics/histograms/enums.xml

@@ -20873,6 +20873,7 @@ Called by update_net_error_codes.py.-->
   <int value="2639" label="WebKitUserModifyReadOnlyEffective"/>
   <int value="2640" label="WebKitUserModifyPlainTextEffective"/>
   <int value="2641" label="CSSAtRuleFontFeatureValues"/>
+  <int value="2642" label="FlexboxSingleLineAlignContent"/>
 </enum>
 
 <enum name="FeaturePolicyFeature">