Revert of [webcrypto] JWK: Updated import(ext, key_ops) and added export of symmetric keys (https://codereview.chromium.org/184043021/)

Reason for revert:
SharedCryptoTest.ImportExportJwkSymmetricKey test failing.
http://build.chromium.org/p/chromium.linux/builders/Linux%20Clang%20%28dbg%29/builds/57322



Original issue's description:
> [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*"
> 
> Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=256399
> 
> Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=257078

TBR=eroman@chromium.org,rsleevi@chromium.org,bryaneyler@google.com,padolph@netflix.com
NOTREECHECKS=true
NOTRY=true
BUG=245025

Review URL: https://codereview.chromium.org/200263002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@257087 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 257087