[headless] Add default browser context to HeadlessBrowser.

headless/public/headless_browser.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef HEADLESS_PUBLIC_HEADLESS_BROWSER_H_
 #define HEADLESS_PUBLIC_HEADLESS_BROWSER_H_
 
 #include <memory>
 #include <string>
 #include <unordered_map>
@@ skipping 38 matching lines @@
   // Returns the HeadlessWebContents associated with the
   // |devtools_agent_host_id| if any.  Otherwise returns null.
   virtual HeadlessWebContents* GetWebContentsForDevToolsAgentHostId(
       const std::string& devtools_agent_host_id) = 0;
 
   // Returns HeadlessBrowserContext associated with the given id if any.
   // Otherwise returns null.
   virtual HeadlessBrowserContext* GetBrowserContextForId(
       const std::string& id) = 0;
 
+  // Allows setting and getting the browser context that DevTools will create
+  // new targets in by default.

[Sami, 2017/01/11 10:54:21]

One side effect of this change is that now you must call
SetDefaultBrowserContext to make createTarget (without an explicit context id)
work. I wonder if we should be a little more helpful and set the first created
context as the default? On one hand I like that you need to say explicitly what
you want, but on the other hand pretty much everyone is going to create a single
context which should be the default. WDYT?

[Eric Seckler, 2017/01/11 11:37:13]

Hehe :) I pondered this a bit as well and went with explicitness in the end.

I think a problem with implicitly making the first context the default one is
that we'd then have to think about how we handle the destruction of that context
-- Would we reset the default context to null, or choose another existing
context (which) as default one? If null, do we set the next context that is
created afterwards to be the default one again?

Whatever we'd choose, it might confuse people that do something more advanced
with the contexts... But I can also see how it streamlines the simple
single-context use case.

Maybe an alternative would be to add a more descriptive error message for
Target.createTarget if no default context is set, which tells them to set one?

[Sami, 2017/01/11 18:20:30]

All good points. I think we should go with making the error message point the
user to the right direction.

+  virtual void SetDefaultBrowserContext(
+      HeadlessBrowserContext* browser_context) = 0;
+  virtual HeadlessBrowserContext* GetDefaultBrowserContext() = 0;
+
   // Returns a task runner for submitting work to the browser file thread.
   virtual scoped_refptr<base::SingleThreadTaskRunner> BrowserFileThread()
       const = 0;
 
   // Returns a task runner for submitting work to the browser io thread.
   virtual scoped_refptr<base::SingleThreadTaskRunner> BrowserIOThread()
       const = 0;
 
   // Returns a task runner for submitting work to the browser main thread.
   virtual scoped_refptr<base::SingleThreadTaskRunner> BrowserMainThread()
@@ skipping 141 matching lines @@
 // the main loop, it will only return after HeadlessBrowser::Shutdown() is
 // called, returning the exit code for the process. It is not possible to
 // initialize the browser again after it has been torn down.
 int HeadlessBrowserMain(
     HeadlessBrowser::Options options,
     const base::Callback<void(HeadlessBrowser*)>& on_browser_start_callback);
 
 }  // namespace headless
 
 #endif  // HEADLESS_PUBLIC_HEADLESS_BROWSER_H_