[webcrypto] Refactor some of the algorithm parameter names.

 (1) Add a suffix of either "Bits" or "Bytes" to all lengths to help with misuse. [*]

 (2) Change how optional parameters are retrieved. Currently they require a getter, to prevent misuse [*]. However this is inconvenient, especially for buffers. Naming them "optional" should be enough to guide proper usage.

[*] There was a bug attempting to do this caught in a codereview.

BUG=245025

Review URL: https://codereview.chromium.org/109393003

git-svn-id: svn://svn.chromium.org/blink/trunk@163691 bbb929c8-8fbe-4397-9dbb-9b2b20218538