CSP hashing performance improvements.

Added a new variant to the StringUTF8Adaptor constructor that allows for the
consumer to specify that the string should be normalized before UTF-8 encoded.
This allows ContentSecurityPolicy to use the fast UTF-8 encoding (without
memcopy) from StringUTF8Adaptor in the common case, while also handling the less
common case.

Review URL: https://codereview.chromium.org/194713012

git-svn-id: svn://svn.chromium.org/blink/trunk@168949 bbb929c8-8fbe-4397-9dbb-9b2b20218538