[IME] Use ExtensionSystem to check extension's availability instead of holding an internal set in ComponentExtensionIMEManagerImpl. This is to fix the problem of xkb extension doesn't expose its resource to chrome-extension://... The root cause of the problem is that ComponentExtensionIMEManagerImpl object is created before login, but ExtensionSystem is initialized after login. Therefore, xkb extension which is loaded before login won't be in ExtensionSystem but ComponentExtensionIMEManagerImpl thinks it's already been loaded.

BUG=351200
TEST=Verified on pixel device.

Review URL: https://codereview.chromium.org/194183003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256201 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256201