I've gotten multiple crashes at shutdown due to my new DCHECKs.  It seems it's possible to hit them with invalid button IDs.  While the DCHECKs won't happen in release builds, what probably will happen is some sort of other problem due to invalid memory access.  So just convert the DCHECKs to conditionals.
I've gotten multiple crashes at shutdown due to my new DCHECKs.  It seems it's possible to hit them with invalid button IDs.  While the DCHECKs won't happen in release builds, what probably will happen is some sort of other problem due to invalid memory access.  So just convert the DCHECKs to conditionals.

BUG=none
TEST=none
Review URL: http://codereview.chromium.org/3155018

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@56226 0039d316-1c4b-4281-b951-d872f2087c98

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