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: de016e52bd170d2d4f2344f9bf92d50478b649e0@71466 0039d316-1c4b-4281-b951-d872f2087c98

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