[headless] Add default browser context to HeadlessBrowser.

headless/public/headless_browser.h

Index: headless/public/headless_browser.h
diff --git a/headless/public/headless_browser.h b/headless/public/headless_browser.h
index 021e91064fbc4e099945b7cc782d1b430afc5a7a..c3043f006ad40216b6313cac3bfbd416898492e1 100644
--- a/headless/public/headless_browser.h
+++ b/headless/public/headless_browser.h
@@ -56,6 +56,12 @@ class HEADLESS_EXPORT HeadlessBrowser {
   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;