GN: Add //build/toolchain/toolchain.gni to exec_script_whitelist
GN: Add //build/toolchain/toolchain.gni to exec_script_whitelist

It's needed for the soon-to-be-rolled chromium_revision.

BUG=webrtc:6665
TBR=ehmaldonado@webrtc.org

Review URL: https://codereview.webrtc.org/2483303002 .

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#14984}

TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc