Add support to WTF::Function to enable partial binding of Functions objects.

In order to support IdleTasks in the blink scheduler, we need to partially bind
Function objects such that some arguments can be build by the caller posting
the task, and other arguments (e.g., the task's deadline) can be bound by the
scheduling code. The CL at https://codereview.chromium.org/410143002 shows this
in use.

BUG=397026,391005

Review URL: https://codereview.chromium.org/410133002

git-svn-id: svn://svn.chromium.org/blink/trunk@178842 bbb929c8-8fbe-4397-9dbb-9b2b20218538