Introduce the fallback theme engine that will be used to paint a fallback UI

Source/Platform/chromium/public/Platform.h

Index: Source/Platform/chromium/public/Platform.h
diff --git a/Source/Platform/chromium/public/Platform.h b/Source/Platform/chromium/public/Platform.h
index ae1f9e8beaabd49ec00171a0cc5fbac7de3c5ea5..ad845ff3a5ca085e8e840e0c1551c74a9d59d8a1 100644
--- a/Source/Platform/chromium/public/Platform.h
+++ b/Source/Platform/chromium/public/Platform.h
@@ -55,6 +55,7 @@ class WebClipboard;
 class WebCompositorSupport;
 class WebCookieJar;
 class WebDiscardableMemory;
+class WebFallbackThemeEngine;
 class WebFileSystem;
 class WebFileUtilities;
 class WebFlingAnimator;
@@ -117,6 +118,8 @@ public:
     // May return null on some platforms.
     virtual WebThemeEngine* themeEngine() { return 0; }
 
+    virtual WebFallbackThemeEngine* fallbackThemeEngine() { return 0; }
+
     // Must return non-null.
     virtual WebHyphenator* hyphenator() { return 0; }