chrome: Enforce matching context for new user and incognito profiles

Selecting a new user or incognito profile via Chrome UI may result in
the creation of a new Profile object. If that occurs, we should find
or create a matching CefBrowserContext and CefRequestContext instead
of reusing an existing mismatched context (e.g. using the original
context for an incognito window would be a mismatch). If a new
CefRequestContext will be created the client can now implement
CefBrowserProcessHandler::GetDefaultRequestContextHandler() to
provide the handler for that context.

To test with a new user profile:
1. Click "Profile" icon, select "Add". Now you have 2+ profiles.
2. Click "Profile" icon, select the other user name to create a new
   window using the other user profile.
3. The new window should launch successfully.

To test with a new incognito profile:
1. Select "New Incognito window" from the 3-dot menu.
2. The new window should launch successfully.

To test DevTools window creation for the new profile:
1. Right-click in the new window, select Inspect.
2. The DevTools window should launch successfully.

libcef/browser/chrome/chrome_browser_context.h

@@ -23,6 +23,9 @@ class ChromeBrowserContext : public CefBrowserContext, public ProfileObserver {
   ChromeBrowserContext(const ChromeBrowserContext&) = delete;
   ChromeBrowserContext& operator=(const ChromeBrowserContext&) = delete;
 
+  // Returns a ChromeBrowserContext for the specified |profile|.
+  static ChromeBrowserContext* GetOrCreateForProfile(Profile* profile);
+
   void InitializeAsync(base::OnceClosure initialized_cb);
 
   // CefBrowserContext overrides.