Make WebMeaningfulLayout an enum class.

Compiling with -Wshadow showed that one of the values conflicted with
Document::ParsingState

Bug: 294205
Change-Id: I4f932767d1962e7615acfefa5af76ba554aabaa7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2461009Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Reviewed-by: Michael Bai <michaelbai@chromium.org>
Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815191}

android_webview/renderer/aw_render_frame_ext.h

@@ -15,7 +15,6 @@
 #include "url/origin.h"
 
 namespace blink {
-enum WebMeaningfulLayout;
 class WebFrameWidget;
 class WebView;
 }

third_party/blink/public/web/web_meaningful_layout.h

@@ -7,7 +7,7 @@
 
 namespace blink {
 
-enum WebMeaningfulLayout {
+enum class WebMeaningfulLayout {
   // Signifies that one of the following things were involved during the layout:
   // * > 200 text characters
   // * > 1024 image pixels