[css-paint-api] Fix UseCounter for PaintWorklet.

The UseCounter being on the partial interface wasn't actually counting
anything. Move to the attribute instead.

Confirmed in V8CSSPartial generated code that this is now doing the
right thing.

Change-Id: Iab0f50535f7a06e4c2ff1172b7c80133554f9ae7
Reviewed-on: https://chromium-review.googlesource.com/923542Reviewed-by: Xida Chen <xidachen@chromium.org>
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537453}

third_party/WebKit/Source/modules/csspaint/CSSPaintWorklet.idl

@@ -7,8 +7,7 @@
 [
     ImplementedAs=CSSPaintWorklet,
     RuntimeEnabled=CSSPaintAPI,
-    MeasureAs=PaintWorklet,
     SecureContext
 ] partial interface CSS {
-    [CallWith=ScriptState] static readonly attribute Worklet paintWorklet;
+    [CallWith=ScriptState,MeasureAs=PaintWorklet] static readonly attribute Worklet paintWorklet;
 };