Revert 60739 (still leaks on ChromiumOS!) - Reland r59972: Eagerly set the IO...

Revert 60739 (still leaks on ChromiumOS!) - Reland r59972: Eagerly set the IO loop used for OCSP.

ChromeOS will create a special Profile for login. Previously, OCSP initialization was done for the "default" ChromeURLRequestContext for each Profile. Since we can have multiple profiles, this causes the initialization (and uninitialization) to happen multiple times, which causes problems for OCSP since we use statics. The solution is to identify the "main" Profile. We create said Profile in BrowserMain. I add an "is_main_" variable to URLRequestContextGetter and URLRequestContext, so that only the "main" URLRequestContext will initialize OCSP.

The only change from r59972 is the location that I set_is_main().  I set it when we set the default request context.  This is because, by the time I had called set_is_main() in BrowserMain, the ChromeURLRequestContextGetter was already created (it gets created during the CreateProfile() function).  So I was too late.

BUG=55940
TEST=Manual

Review URL: http://codereview.chromium.org/3391028

TBR=willchan@chromium.org

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=60753