UseCounter: Gather web-compat data for changing <dialog> behaviour.

As per:
w3c/csswg-drafts#4645

This adds two different use-counters:
1) Determine how often we query a non-zero scroll offset.
2) Determine how often a dialog is larger than the viewport.

Change-Id: I2a3532512ad29ad22e997073ae9eed80612bf4ee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2258213
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Reviewed-by: Aleks Totic <atotic@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#781149}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: f4efbd052053ec693a6fe2298067ba400e14ec59

blink/public/mojom/web_feature/web_feature.mojom

@@ -2663,6 +2663,8 @@ enum WebFeature {
   kFetchUploadStreaming = 3326,
   kWebkitLineClampWithoutWebkitBox = 3327,
   kWebBluetoothGetDevices = 3328,
+  kDialogWithNonZeroScrollOffset = 3329,
+  kDialogHeightLargerThanViewport = 3330,
 
   // Add new features immediately above this line. Don't change assigned
   // numbers of any item, and don't reuse removed slots.

blink/renderer/core/layout/ng/ng_absolute_utils.cc

@@ -369,15 +369,21 @@ base::Optional<LayoutUnit> ComputeAbsoluteDialogYPosition(
     return base::nullopt;
   }
 
-  auto* scrollable_area = dialog.GetDocument().View()->LayoutViewport();
+  auto& document = dialog.GetDocument();
+  auto* scrollable_area = document.View()->LayoutViewport();
   LayoutUnit top =
       LayoutUnit((dialog.Style()->GetPosition() == EPosition::kFixed)
                      ? 0
                      : scrollable_area->ScrollOffsetInt().Height());
 
-  int visible_height = dialog.GetDocument().View()->Height();
+  if (top)
+    UseCounter::Count(document, WebFeature::kDialogWithNonZeroScrollOffset);
+
+  int visible_height = document.View()->Height();
   if (height < visible_height)
     top += (visible_height - height) / 2;
+  else if (height > visible_height)
+    UseCounter::Count(document, WebFeature::kDialogHeightLargerThanViewport);
   dialog_node->SetCentered(top);
   return top;
 }