Merge 58280 - Allow download Chrome page to set Chrome as default even if it ...

Merge 58280 - Allow download Chrome page to set Chrome as default even if it is not a new install.

Read "make_default_for_user" preference from master_preferences file and set Chrome to default in installer if we are updating Chrome instead of installing a new version.
The problem here is that now that we set the default browser preference from a web page, we are using master_preferences to communicate this desire to the browser.
Because the user already has Chrome installed on his/her machine, the master_preferences file will not be written to their machine.
Instead, we check for this preference in the installer itself, and set Chrome to be default for the user here.

BUG=53656
TEST=Have Chrome installed on your machine, but *not* default browser.  Run setup.exe to install a new version of Chrome, with a master_preferences file that includes the distribution preference "make_default_for_user":true. (This reflects what happens when the user downloads Chrome with the "make default" option checked).
Chrome should be default browser.

Review URL: http://codereview.chromium.org/3342002

TBR=mirandac@chromium.org

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=58288