Unregister previous ChromeFrame UA strings registered under the PostPlatform key while registering a new ChromeFrame UA string. This ensures that we don't end up with a number of duplicated chrome frame UA strings each pointing to a different version.
Unregister previous ChromeFrame UA strings registered under the PostPlatform key while registering a new ChromeFrame UA string.
This ensures that we don't end up with a number of duplicated chrome frame UA strings each pointing to a different version.

This should hopefully fix the InstallFlowTest.

BUG=none
TEST=Covered by existing ChromeFrame InstallFlowTest

Review URL: http://codereview.chromium.org/6355001

git-svn-id: http://src.chromium.org/svn/trunk/src@71466 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: 3d79ae827355b390ba50c38f3f6644791c77044f
meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser