base::UnguessableToken seems to fit the Google C++ style guide's
criterion on overloading << as a streaming operator. Notably:
-- It represents a value.
-- There is an easy human-readable string reprensetation of the value
(0 padded HEX).

This change adds an overload to the << operator, as syntactic sugar.

BUG=648441
TEST= Modified a UT to test the operator.

Review-Url: https://codereview.chromium.org/2353663003
Cr-Commit-Position: refs/heads/master@{#419647}