Build API: Add USE flag passing support to the API.

BUG=chromium:959931
TEST=run_tests
Cq-Depend: chromium:1595968

Change-Id: I4e292767f5a827e72912aa8379817b7ff6aaaae1
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/1596313
Tested-by: Alex Klein <saklein@chromium.org>
Reviewed-by: Evan Hernandez <evanhernandez@chromium.org>
Commit-Queue: Alex Klein <saklein@chromium.org>
diff --git a/api/controller/sysroot.py b/api/controller/sysroot.py
index 5e6f82e..04aa31e 100644
--- a/api/controller/sysroot.py
+++ b/api/controller/sysroot.py
@@ -93,9 +93,10 @@
   if not target_sysroot.IsToolchainInstalled():
     cros_build_lib.Die('Toolchain must first be installed.')
 
+  use_flags = [u.flag for u in input_proto.use_flags]
   build_packages_config = sysroot.BuildPackagesRunConfig(
       event_file=event_file, usepkg=not compile_source,
-      install_debug_symbols=True, packages=packages)
+      install_debug_symbols=True, packages=packages, use_flags=use_flags)
 
   try:
     sysroot.BuildPackages(build_target, target_sysroot, build_packages_config)