linux: plumb shift-reload down into new shift-reload API

Currently Linux-only, but the Mac/Win bits should now be trivial.

While I was add it, I tweaked a NavigationController function
that took a bare boolean into an enum to make the call sites more
explicit about what they're doing.

(In most places I added new functions that call into a shared
backing function; this is so I don't need to change every single
caller of e.g. Reload() to pass through an extra flag that will
be the same for almost every caller.)

BUG=1906
TEST=visit astronomy picture of the day; hit reload, picture pops
up quickly; hit shift-reload, picture loads slowly

Review URL: http://codereview.chromium.org/594063

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@39430 0039d316-1c4b-4281-b951-d872f2087c98
9 files changed