Enable in-tree libc++ builds on iOS

Binary size regression of about 109kB is expected -- please add
thomasanderson@ and thakis@ to the bug.

BUG=945054
R=thakis

Change-Id: Iaca4178e3b23b55a36af55d119d2eda2b76ef71d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1534897
Commit-Queue: Thomas Anderson <[email protected]>
Auto-Submit: Thomas Anderson <[email protected]>
Reviewed-by: Justin Cohen <[email protected]>
Reviewed-by: Rohit Rao <[email protected]>
Reviewed-by: Nico Weber <[email protected]>
Cr-Commit-Position: refs/heads/master@{#645001}
4 files changed