Fixed blank tab is shown after attaching split view

fix brave/brave-browser#41852

When split view transfers to another browser, its web contents should
be cleared before attached to target browser.
It's done by SplitView::UpdateContentsWebViewVisual().

So far, we posted it to next turn when we break tile.
If we break a tile by unsplit, it doesn't cause any issue because
any other secondary web view doesn't host that secondary web contents.

However, if it's attached to another browser window, it's another story.
When we detach a split view from browser A and attach to browser B,
browser A breaks a tile after detached. And then, they are attached to
browser B and we make them as a split view. If we don't clear secondary
web contents after break the tile synchronously, browser A and browser
B's secondary web view has same web contents till posed
`UpdateContentsWebViewVisual(). This could cause unexpected behavior
like above issue. So, secondary web view should be cleared synchronously
after breaking a tile.

browser/ui/views/frame/brave_browser_view.cc

@@ -856,14 +856,12 @@ void BraveBrowserView::OnTileTabs(const TabTile& tile) {
   UpdateContentsWebViewVisual();
 }
 
-void BraveBrowserView::OnWillBreakTile(const TabTile& tile) {
+void BraveBrowserView::OnDidBreakTile(const TabTile& tile) {
   if (!IsActiveWebContentsTiled(tile)) {
     return;
   }
 
-  base::SequencedTaskRunner::GetCurrentDefault()->PostTask(
-      FROM_HERE, base::BindOnce(&BraveBrowserView::UpdateContentsWebViewVisual,
-                                weak_ptr_.GetWeakPtr()));
+  UpdateContentsWebViewVisual();
 }
 
 void BraveBrowserView::OnSwapTabsInTile(const TabTile& tile) {

browser/ui/views/frame/brave_browser_view.h

@@ -131,7 +131,7 @@ class BraveBrowserView : public BrowserView,
 
   // SplitViewBrowserDataObserver:
   void OnTileTabs(const TabTile& tile) override;
-  void OnWillBreakTile(const TabTile& tile) override;
+  void OnDidBreakTile(const TabTile& tile) override;
   void OnSwapTabsInTile(const TabTile& tile) override;
 
   views::WebView* secondary_contents_web_view() {

browser/ui/views/frame/split_view_browsertest.cc

@@ -82,7 +82,6 @@ IN_PROC_BROWSER_TEST_F(SplitViewBrowserTest,
       split_view_data->IsTabTiled(tab_strip_model().GetTabHandleAt(0)));
 
   // Then, the secondary web view should become hidden
-  base::RunLoop().RunUntilIdle();
   EXPECT_FALSE(secondary_contents_view().GetVisible());
 }