Allow HTMLPlugInElement to host UA shadow DOM.

Source/core/css/resolver/StyleAdjuster.cpp

Index: Source/core/css/resolver/StyleAdjuster.cpp
diff --git a/Source/core/css/resolver/StyleAdjuster.cpp b/Source/core/css/resolver/StyleAdjuster.cpp
index 6225ed6d32fcef01ac00058f5eeaf32fbecc80a4..01b0857a3e2e95ba27a5e8c6f42dbe2f07299594 100644
--- a/Source/core/css/resolver/StyleAdjuster.cpp
+++ b/Source/core/css/resolver/StyleAdjuster.cpp
@@ -42,6 +42,7 @@
 #include "core/html/HTMLTextAreaElement.h"
 #include "core/frame/FrameView.h"
 #include "core/frame/Settings.h"
+#include "core/rendering/RenderReplaced.h"
 #include "core/rendering/RenderTheme.h"
 #include "core/rendering/style/GridPosition.h"
 #include "core/rendering/style/RenderStyle.h"
@@ -383,6 +384,16 @@ void StyleAdjuster::adjustStyleForTagName(RenderStyle* style, RenderStyle* paren
 
     if (isHTMLPlugInElement(element)) {
         style->setRequiresAcceleratedCompositingForExternalReasons(toHTMLPlugInElement(element).shouldAccelerate());
+
+        // Plugins should get the standard replaced width/height instead of 'auto'.
+        // Replaced renderers get this for free, and fallback content doesn't count.
+        if (toHTMLPlugInElement(element).usePlaceholderContent()) {
+            if (style->width().isAuto())
+                style->setWidth(Length(RenderReplaced::defaultWidth, Fixed));
+            if (style->height().isAuto())
+                style->setHeight(Length(RenderReplaced::defaultHeight, Fixed));
+        }
+
         return;
     }
 }