We should add this define to external_config because it is used in
public include v8.h (e.g.: https://cs.chromium.org/chromium/src/v8/include/v8.h?l=272&rcl=5cd6565d5ad06a8cb5a1d9d502d15a54e4fa5bbe)

Change-Id: I795a3de448029e34033cf8f83094bdea3590bbb9
Reviewed-on: https://chromium-review.googlesource.com/654876Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47901}