Fix crash when de-referencing the WebFrameWidget.

third_party/WebKit/Source/web/WebViewImpl.cpp

Index: third_party/WebKit/Source/web/WebViewImpl.cpp
diff --git a/third_party/WebKit/Source/web/WebViewImpl.cpp b/third_party/WebKit/Source/web/WebViewImpl.cpp
index 8a38208b82174a374b576187f2450ca2c8b40ac8..b982e288a6868b086d9008056ceccb60efb000e7 100644
--- a/third_party/WebKit/Source/web/WebViewImpl.cpp
+++ b/third_party/WebKit/Source/web/WebViewImpl.cpp
@@ -2538,15 +2538,23 @@ void WebViewImpl::willCloseLayerTreeView() {
 }
 
 void WebViewImpl::didAcquirePointerLock() {
-  mainFrameImpl()->frameWidget()->didAcquirePointerLock();
+  if (mainFrameImpl())
+    mainFrameImpl()->frameWidget()->didAcquirePointerLock();
 }
 
 void WebViewImpl::didNotAcquirePointerLock() {
-  mainFrameImpl()->frameWidget()->didNotAcquirePointerLock();
+  if (mainFrameImpl())
+    mainFrameImpl()->frameWidget()->didNotAcquirePointerLock();
 }
 
 void WebViewImpl::didLosePointerLock() {
-  mainFrameImpl()->frameWidget()->didLosePointerLock();
+  // Make sure that the main frame wasn't swapped-out when the pointer lock is
+  // lost. There's a race that can happen when a pointer lock is requested, but
+  // the browser swaps out the main frame while the pointer lock request is in
+  // progress. This won't be needed once the main frame is refactored to not use
+  // the WebViewImpl as its WebWidget.
+  if (mainFrameImpl())
+    mainFrameImpl()->frameWidget()->didLosePointerLock();
 }
 
 // TODO(ekaramad):This method is almost duplicated in WebFrameWidgetImpl as