base::Hash: Minor improvements to documentation and testing.

Renamed the char* argument to 'data' and the string argument to 'str'.
Cleaned up the comments on the char* overload (avoid using
string-specific terms and mentioning null byte). Mentioned that this
algorithm is also in Blink.

Added a test case testing that null bytes cause the hash to change.

BUG=347393

Review URL: https://codereview.chromium.org/182253007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255269 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255269