Change GN defaults for toolchains for CrOS.

Change GN defaults for toolchains for CrOS.

Previously we would attempt to use clang for the host toolchain
by default regardless of what is_clang was set to. However,
we really only want to use clang for the host toolchain on Android;
on CrOS, we typically don't have clang for either host or target,
and so if is_clang is false for the target it should probably also
be false for the host toolchain.

This CL changes the defaults accordingly.

It also adds a comment to the sysroot variables to make it clearer
that target_sysroot and use_sysroot can both be used; this is
needed so that you can set target_sysroot= for the target and then
set use_sysroot=false for the other toolchains (so that the host
toolchain doesn't attempt to use a sysroot it can't use).

R=brettw@chromium.org, jochen@chromium.org
BUG=608596

[jochen (gone - plz use gerrit), 2016-05-11 08:56:19]

lgtm

[brettw, 2016-05-11 20:29:10]

lgtm

[Dirk Pranke, 2016-05-17 21:51:59]

Description was changed from

==========
Change GN defaults for toolchains for CrOS.

Previously we would attempt to use clang for the host toolchain
by default regardless of what is_clang was set to. However,
we really only want to use clang for the host toolchain on Android;
on CrOS, we typically don't have clang for either host or target,
and so if is_clang is false for the target it should probably also
be false for the host toolchain.

This CL changes the defaults accordingly.

It also adds a comment to the sysroot variables to make it clearer
that target_sysroot and use_sysroot can both be used; this is
needed so that you can set target_sysroot= for the target and then
set use_sysroot=false for the other toolchains (so that the host
toolchain doesn't attempt to use a sysroot it can't use).

R=brettw@chromium.org, jochen@chromium.org
BUG=608596
==========

to

==========
Change GN defaults for toolchains for CrOS.

Previously we would attempt to use clang for the host toolchain
by default regardless of what is_clang was set to. However,
we really only want to use clang for the host toolchain on Android;
on CrOS, we typically don't have clang for either host or target,
and so if is_clang is false for the target it should probably also
be false for the host toolchain.

This CL changes the defaults accordingly.

It also adds a comment to the sysroot variables to make it clearer
that target_sysroot and use_sysroot can both be used; this is
needed so that you can set target_sysroot= for the target and then
set use_sysroot=false for the other toolchains (so that the host
toolchain doesn't attempt to use a sysroot it can't use).

R=brettw@chromium.org, jochen@chromium.org
BUG=608596
==========

[Dirk Pranke, 2016-05-17 21:52:35]

Abandoning this in favor of https://codereview.chromium.org/1983613002/ - we
need the more general approach in that CL, I think.