Expose a way to set a view's base background color.

Source/web/WebViewImpl.cpp

Index: Source/web/WebViewImpl.cpp
diff --git a/Source/web/WebViewImpl.cpp b/Source/web/WebViewImpl.cpp
index b809f05932b0600c34a375563b90e8478061b572..fe17d3770d187610a482b46a0acc642632796b2b 100644
--- a/Source/web/WebViewImpl.cpp
+++ b/Source/web/WebViewImpl.cpp
@@ -428,6 +428,7 @@ WebViewImpl::WebViewImpl(WebViewClient* client)
     , m_showDebugBorders(false)
     , m_continuousPaintingEnabled(false)
     , m_showScrollBottleneckRects(false)
+    , m_baseBackgroundColor(Color::white)
 {
     Page::PageClients pageClients;
     pageClients.chromeClient = &m_chromeClientImpl;
@@ -2462,11 +2463,11 @@ WebColor WebViewImpl::backgroundColor() const
     if (isTransparent())
         return Color::transparent;
     if (!m_page)
-        return Color::white;
+        return m_baseBackgroundColor;
     FrameView* view = m_page->mainFrame()->view();
     StyleColor backgroundColor = view->documentBackgroundColor();
     if (!backgroundColor.isValid())
-        return Color::white;
+        return m_baseBackgroundColor;
     return backgroundColor.rgb();
 }
 
@@ -3579,6 +3580,19 @@ bool WebViewImpl::isTransparent() const
     return m_isTransparent;
 }
 
+void WebViewImpl::setBaseBackgroundColor(WebColor color)
+{
+    if (m_baseBackgroundColor == color)
+        return;
+
+    m_baseBackgroundColor = color;
+
+    m_page->mainFrame()->view()->setBaseBackgroundColor(color);
+
+    if (m_layerTreeView)
+        m_layerTreeView->setBackgroundColor(backgroundColor());
+}
+
 void WebViewImpl::setIsActive(bool active)
 {
     if (page() && page()->focusController())