chrome: Support Chrome and DevTools command execution (fixes #3282, fixes #3600)

Add new CefBrowserHost::[Can]ExecuteChromeCommand methods for executing
arbitrary Chrome commands.

Add support for existing CefBrowserHost::ShowDevTools, CloseDevTools and
HasDevTools methods.

DevTools windows now support the same Views callbacks as normal popup
windows with the new CefLifeSpanHandler::OnBeforeDevToolsPopup callback
as the DevTools-specific equivalent of OnBeforePopup.

Always create DevTools as an undocked window to support use of
ShowDevTools with default Chrome browser windows.

To test:

Run `ceftests --enable-chrome-runtime [--use-views]
     --gtest_filter=V8Test.OnUncaughtExceptionDevTools`

OR:

1. Run `cefclient --enable-chrome-runtime [--use-native]`
2. Select "Show DevTools", "Close DevTools" or "Inspect" from the
   right-click menu.
3. Notice that the DevTools window is Views-hosted (or native-hosted)
   and works as expected.

Add --use-default-popup to get a default styled popup in step 3.

libcef/browser/request_context_impl.h

@@ -38,6 +38,15 @@ class CefRequestContextImpl : public CefRequestContext {
   static CefRefPtr<CefRequestContextImpl> GetOrCreateForRequestContext(
       CefRefPtr<CefRequestContext> request_context);
 
+  // Returns the BrowserContext for the specified |request_context|. Will return
+  // the global BrowserContext if |request_context| is NULL.
+  static content::BrowserContext* GetBrowserContext(
+      CefRefPtr<CefRequestContext> request_context);
+
+  // Returns the Profile for the specified |request_context|. Will return the
+  // global Profile if |request_context| is NULL.
+  static Profile* GetProfile(CefRefPtr<CefRequestContext> request_context);
+
   // Verify that the browser context can be directly accessed (e.g. on the UI
   // thread and initialized).
   bool VerifyBrowserContext() const;