Add some SmokeTests for WebCrypto.

This selection wasn't very scientific. These were the guidelines that I used:

  - Tests that are fast
  - Covers some different types of operations
  - Runs on all platforms [1]

[1] The RSA operations are not supported on all the Linux bots (depends on the version of system NSS library). This ruled out using any tests that import private RSA keys.

BUG=399038

Review URL: https://codereview.chromium.org/466993003

git-svn-id: svn://svn.chromium.org/blink/trunk@184108 bbb929c8-8fbe-4397-9dbb-9b2b20218538