In https://codereview.chromium.org/2499313003, which sets the user gesture bit at navigation handle creation time, it was discovered that the user gesture bit can sometimes be inconsistent during the lifetime of a NavigationHandle. See bug 667070 for a specific example of why this can happen. We would like to fix this, then DCHECK that the gesture bit received in DidCommitNavigation is consistent with the bit set at construction time.