Change default Omaha installsource param from "" to "ondemand"

Change default Omaha installsource param from "" to "ondemand"

This will let Omaha know the difference between new, on-demand installs
that are manually initiated from the webstore, and the background
autoupdate checks that we do for already installed extensions/apps.

BUG=249187
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=208048

[asargent_no_longer_on_chrome, 2013-06-21 21:57:12]

waffles: please review from Omaha's perspective

[waffles, 2013-06-21 22:01:02]

lgtm

[asargent_no_longer_on_chrome, 2013-06-21 22:04:06]

Matt: please rubberstamp

[Matt Perry, 2013-06-21 22:05:47]

lgtm

[commit-bot: I haz the power, 2013-06-21 22:07:27]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/asargent@chromium.org/17524010/1

[commit-bot: I haz the power, 2013-06-22 02:45:35]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/asargent@chromium.org/17524010/1

[commit-bot: I haz the power, 2013-06-22 03:08:37]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/asargent@chromium.org/17524010/1

[commit-bot: I haz the power, 2013-06-22 03:21:25]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/asargent@chromium.org/17524010/1

[commit-bot: I haz the power, 2013-06-22 03:36:19]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/asargent@chromium.org/17524010/1

[commit-bot: I haz the power, 2013-06-22 03:43:53]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/asargent@chromium.org/17524010/1

[commit-bot: I haz the power, 2013-06-22 14:53:23]

Change committed as 208048