WebCrypto: Add more operations to the platform API.

Adds methods for encrypt(), decrypt() and sign()

BUG=245025

Review URL: https://chromiumcodereview.appspot.com/21016005

git-svn-id: svn://svn.chromium.org/blink/trunk@155224 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Remove String::adopt(Vector<UChar>)

Adopting a Vector<UChar> isn't an efficient way to build a string. There are
only two callers, so we should remove this API before we grow more. I've moved
the callers over to StringBuilder and StringBuffer. There was one caller in
EventSource that I moved to copying the characters rather than adopting them.

R=eseidel

Review URL: https://chromiumcodereview.appspot.com/21262003

git-svn-id: svn://svn.chromium.org/blink/trunk@155225 bbb929c8-8fbe-4397-9dbb-9b2b20218538