Revert 256399 "[webcrypto] JWK: Updated import(ext, key_ops) and..."

This caused ImportExportJwkSymmetricKey test failure on dbg builds, e.g.
http://build.chromium.org/p/chromium.linux/builders/Linux%20Clang%20%28dbg%29/builds/57242

as assertion fail.  This may not have been caught by rel builds on trybots.

> [webcrypto] JWK: Updated import(ext, key_ops) and added export of symmetric keys
> 
> See https://www.w3.org/Bugs/Public/show_bug.cgi?id=23796
> 
> BUG=245025
> TEST=content_unittests --gtest_filter="SharedCryptoTest*"
> 
> Review URL: https://codereview.chromium.org/184043021

TBR=padolph@netflix.com

Review URL: https://codereview.chromium.org/196683002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256415 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256415