Fix crash in BluetoothPairingUI.

Due to recent refactors introduced in
https://codereview.chromium.org/144983002,
CoreChromeOSOptionsHandler::GetLocalizedValues constructs a Profile
object. The crash was caused by a NULL pointer being passed to
Profile::FromWebUI because CoreChromeOSOptionsHandler::web_ui() wasn't set.
This CL addresses this by making sure that WebUI::AddMessageHandler is called
before CoreChromeOSOptionsHandler::GetLocalizedValues, so that the web_ui
pointer gets set before being referenced.

BUG=341671
TEST=Tested pairing on peppy and on Goobuntu with chromeos=1.
R=jennyz@chromium.org, xiyuan@chromium.org

Review URL: https://codereview.chromium.org/157703004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250042 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250042
Change WorkerHostMsg_WorkerContextDestroyed to control message.

BUG=329786

Review URL: https://codereview.chromium.org/138793004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250043 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250043