blink: fix AltTextContentData::Clone()

It should return AltTextContentData, not TextContentData.

I've tagged this with bug 1087667 as a patch I'm working on for
that bug triggers calling this.

BUG=1087667
TEST=none

Change-Id: I3e04f8689f3df5a17c94abeaacad05b5e6962a8c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2466058Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816333}

third_party/blink/renderer/core/style/content_data.h

@@ -175,7 +175,7 @@ class AltTextContentData final : public ContentData {
 
  private:
   ContentData* CloneInternal() const override {
-    return MakeGarbageCollected<TextContentData>(GetText());
+    return MakeGarbageCollected<AltTextContentData>(GetText());
   }
   String text_;
 };