Refactor RSA key generation for WebCrypto's NSS implementation.

This refactor extracts common functionality to simplify adding the equivalent OpenSSL implementation:

 * Extracts the conversion of public exponent from a big integer to an unsigned integer
 * Restricts key sizes to multiples of 8, between 256 and 16K. This is the more restrictive range supported by NSS.

BUG=389317

Review URL: https://codereview.chromium.org/401233004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@285532 0039d316-1c4b-4281-b951-d872f2087c98
6 files changed