Avoid clobbering argument count when loading instance type.

TBR=bmeurer@chromium.org, ivica.bogosavljevic@imgtec.com

Bug: chromium:706642
Change-Id: I82ceb6f1270420ec683f0659f9433795562ab1b4
Reviewed-on: https://chromium-review.googlesource.com/471872Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44497}