Convert ContentMain to take a struct instead of parameters that vary depending on the platform. This helps reduce ifdef mess, and makes it easier to add extra optional parameters. In a followup cl, I'll move the ui_task parameter from MainFunctionParams to ContentMainParams.

BUG=350550
R=sky@chromium.org

Review URL: https://codereview.chromium.org/190853004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255866 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255866