BrowserMain() refactoring, part 2.

Add "MainMessageLoopStart()" and related platform methods to handle tasks
directly tied to the start of the main message loop.

BUG=none
TEST=everything still works

Review URL: http://codereview.chromium.org/2931007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@53783 0039d316-1c4b-4281-b951-d872f2087c98
8 files changed