The implementation of MemorySize with RelocatableInt32Constants is
problematic if MemorySize is placed close to a GrowMemory instruction in
the code. The use of a runtime function guarantees that the order in
which MemorySize and GrowMemory is executed is correct.

R=titzer@chromium.org
BUG=chromium:651961
TEST=mjsunit/regress/wasm/regression-651961

Review-Url: https://codereview.chromium.org/2386183004
Cr-Commit-Position: refs/heads/master@{#39972}