Use root_out_dir for lib_dirs when building with a custom libc++.

Use root_out_dir for lib_dirs when building with a custom libc++.

Currently, using a custom libc++ adds root_build_dir, the root of the
build, to lib_dirs. When building using a non-default toolchain, this
incorrectly adds the default toolchain's output directory to lib_dirs.
This CL corrects this issue by adding root_out_dir, the root of the
build for the current toolchain, to lib_dirs instead.

BUG=573288

[Sam McNally, 2016-05-17 05:31:11]

Description was changed from

==========
Use root_out_dir for lib_dirs when building with a custom libc++.

Currently, using a custom libc++ adds root_build_dir, which always
expands to the root of the build, to lib_dirs. When building using a
non-default toolchain, this incorrectly adds the default toolchain's
output directory to lib_dirs. This CL corrects this issue by using
root_out_dir, which expands to the root of the build for the current
toolchain, to lib_dirs.

BUG=573288
==========

to

==========
Use root_out_dir for lib_dirs when building with a custom libc++.

Currently, using a custom libc++ adds root_build_dir, the root of the
build, to lib_dirs. When building using a non-default toolchain, this
incorrectly adds the default toolchain's output directory to lib_dirs.
This CL corrects this issue by adding root_out_dir, the root of the
build for the current toolchain, to lib_dirs instead.

BUG=573288
==========

[Sam McNally, 2016-05-17 05:37:05]

sammc@chromium.org changed reviewers:
+ glider@chromium.org