GitLab

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: svn://svn.chromium.org/chrome/trunk/src@71466 0039d316-1c4b-4281-b951-d872f2087c98