WTF: Make String's pretty-printing from gtest prettier.

This patch extends the content of WTFTestHelpers and updates String's
pretty-printing logic used by gtest so it would make more sense for
developers.

Most notably, the pretty printer now differentiates a null string from an empty
string, and also it is able to print control characters escaped.

Unit-test only patch; no behavior change expected.

BUG=none

Review URL: https://codereview.chromium.org/98153002

git-svn-id: svn://svn.chromium.org/blink/trunk@162953 bbb929c8-8fbe-4397-9dbb-9b2b20218538