Handle plugin instance crash in ContentDecryptorDelegate.

BUG=chrome-os-partner:17801
TEST=Kill Widevine CDM process and we get decode error.

Review URL: https://codereview.chromium.org/116443009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243762 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243762
Refactoring around descriptor.  The descriptor representing InputMethodEngine was previously generated in InpuMethodManager.  This patch makes InputMethodEngine generates its own descriptor and manage it.

BUG=N/A

Review URL: https://codereview.chromium.org/119133003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243763 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243763