Calling pkg-config directly doesn't work when building
with a sysroot image as it will looks for host libraries
rather than those within the sysroot.

BUG=chromium:569947

Change-Id: I6f3e63c1816cd9393110cb3060744a69a93c9a2e
Reviewed-on: https://chromium-review.googlesource.com/318771Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tryjob-Request: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Sam Clegg <sbc@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>