Adds a fail-safe logic to IMEBridgeImpl.

Makes IMEBridgeImpl::SetCurrentEngineHandlerById(engine_id) more robust and not assume that the given id should be valid, though it should be valid.

BUG=341283
TEST=none

Review URL: https://codereview.chromium.org/151223004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249669 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249669