This should give more potential for inlining the function. Makes the
non-parameterized set and assign private in OffsetBindingPointer to
prevent user errors.

Bug: angleproject:2891
Change-Id: I0d581a96154c0cd33a93b06ef4a3c162809a8208
Reviewed-on: https://chromium-review.googlesource.com/c/1286378
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>