Handle WebContents going away in content settings bubbles.

This fixes null dereference crashes when tabs close with open bubbles.

BUG=622973

Review-Url: https://codereview.chromium.org/2103733002
Cr-Commit-Position: refs/heads/master@{#402610}

chrome/browser/ui/content_settings/content_setting_bubble_model.cc

@@ -978,7 +978,9 @@ ContentSettingMixedScriptBubbleModel::ContentSettingMixedScriptBubbleModel(
 }
 
 void ContentSettingMixedScriptBubbleModel::OnCustomLinkClicked() {
-  DCHECK(web_contents());
+  if (!web_contents())
+    return;
+
   web_contents()->SendToAllFrames(
       new ChromeViewMsg_SetAllowRunningInsecureContent(MSG_ROUTING_NONE, true));
   web_contents()->GetMainFrame()->Send(new ChromeViewMsg_ReloadFrame(
@@ -1083,6 +1085,9 @@ void ContentSettingRPHBubbleModel::OnRadioClicked(int radio_index) {
 }
 
 void ContentSettingRPHBubbleModel::OnDoneClicked() {
+  if (!web_contents())
+    return;
+
   // The user has one chance to deal with the RPH content setting UI,
   // then we remove it.
   TabSpecificContentSettings::FromWebContents(web_contents())->
@@ -1094,6 +1099,9 @@ void ContentSettingRPHBubbleModel::OnDoneClicked() {
 }
 
 void ContentSettingRPHBubbleModel::RegisterProtocolHandler() {
+  if (!web_contents())
+    return;
+
   // A no-op if the handler hasn't been ignored, but needed in case the user
   // selects sequences like register/ignore/register.
   registry_->RemoveIgnoredHandler(pending_handler_);
@@ -1104,6 +1112,9 @@ void ContentSettingRPHBubbleModel::RegisterProtocolHandler() {
 }
 
 void ContentSettingRPHBubbleModel::UnregisterProtocolHandler() {
+  if (!web_contents())
+    return;
+
   registry_->OnDenyRegisterProtocolHandler(pending_handler_);
   TabSpecificContentSettings::FromWebContents(web_contents())->
       set_pending_protocol_handler_setting(CONTENT_SETTING_BLOCK);
@@ -1111,6 +1122,9 @@ void ContentSettingRPHBubbleModel::UnregisterProtocolHandler() {
 }
 
 void ContentSettingRPHBubbleModel::IgnoreProtocolHandler() {
+  if (!web_contents())
+    return;
+
   registry_->OnIgnoreRegisterProtocolHandler(pending_handler_);
   TabSpecificContentSettings::FromWebContents(web_contents())->
       set_pending_protocol_handler_setting(CONTENT_SETTING_DEFAULT);