[webcrypto] Remove a special case for AES-KW wrapping/unwrapping.

[webcrypto] Remove a special case for AES-KW wrapping/unwrapping.

Now the same export+encrypt / decrypt+import codepath is used as for other wrapping algorithms.

The new code is less efficient (creates a temporary raw key), however simplifies things, and allows wrapping to be done on non-raw keys (however because of size constraints is likely to fail).

BUG=373544, 245025
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=276890

[Ryan Sleevi, 2014-06-12 19:09:48]

lgtm

[eroman, 2014-06-12 19:17:00]

The CQ bit was checked by eroman@chromium.org

[commit-bot: I haz the power, 2014-06-12 19:18:00]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/eroman@chromium.org/335463002/40001

[eroman, 2014-06-12 19:50:13]

The CQ bit was checked by eroman@chromium.org

[commit-bot: I haz the power, 2014-06-12 19:53:03]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/eroman@chromium.org/335463002/60001

[commit-bot: I haz the power, 2014-06-13 02:31:11]

Change committed as 276890