size_t comparison warning in Win32.

src\libangle\renderer\d3d\programd3d.cpp(2084) : warning C4018: '<' : signed/unsigned mismatch

BUG=510151
This reverts commit bf3f780c.

Change-Id: I5565b697ca29270375049f3e855437fa19157de9
Reviewed-on: https://chromium-review.googlesource.com/292331Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>