Removal of keybinding preferences should be limited to current working OS platform only.

Fix the problem: when an user use the same account to sync on different OS
platforms, the keybinding assigned on one platform will be reverted.

BUG=322727, 316567
TEST=0. Use the same account to log in Chrome on different OS platforms.
1. Install extensions that can use chrome.commands, such as Google Dictionary[0].
2. Go to Keyboard Shortcuts for Extensions of chrome://extensions.
3. Assign a shortcut to the "Activate the extension".
4. Do the same thing on another OS platform.
5. Wait for the sync (Should finish in a short time).
6. Restart the chrome and observe the "Keyboard Shortcuts for Extensions".
7. Shortcuts should keep assigned.

[0]: https://chrome.google.com/webstore/detail/google-dictionary-by-goog/mgijmajocgfcbeboacabfgobmjgjcoja

Review URL: https://codereview.chromium.org/82763011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237113 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237113