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This unifies the creation of GTK widgets and makes it happen in response to the infobar being shown. Currently, this generally happens at construction. Once the new ownership model lands, infobars will not yet be owned at construction time, and since widget creation includes getting a theme service from the owner, that needs to move out of the construction phase.

Incidentally, this also makes the GTK native object creation happen at the same time as the views native object creation, not that that matters much.

BUG=none
TEST=none

Review URL: https://codereview.chromium.org/14327008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@194707 0039d316-1c4b-4281-b951-d872f2087c98