Default to using Chrome's clang for CrOS's host & v8_snapshot toolchains.

All uses of Simple Chrome currently already do this:
https://cs.chromium.org/chromium/src/third_party/chromite/cli/cros/cros_chrome_sdk.py?rcl=8b42807c6f1e5b2a046695f363227138f56db6cb&l=999

While the ebuild explicitly overwrites the host toolchain with the host
toolchain in the chroot:
https://chromium.googlesource.com/chromiumos/overlays/chromiumos-overlay/+/aeeb6096850b8fe779d87b83a5498160e25a3ffa/chromeos-base/chromeos-chrome/chromeos-chrome-9999.ebuild#907

So this should have no effect on ebuilds of chrome, while letting us
delete parts of simeplchrome's chromite code.

Bug: 937821
Change-Id: Ic589c948407c6dd34bd67e95520cc05ed84740b3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1976284Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: George Burgess <gbiv@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#726579}

build/toolchain/cros/BUILD.gn

@@ -107,6 +107,9 @@ gcc_toolchain("host") {
   if (cxx != get_path_info(cxx, "file") && string_replace(cxx, " ", "") == cxx) {
     cxx = rebase_path(cxx, root_build_dir)
   }
+  if (ar != get_path_info(ar, "file") && string_replace(ar, " ", "") == ar) {
+    ar = rebase_path(ar, root_build_dir)
+  }
   ld = cxx
   if (cros_host_ld != "") {
     ld = cros_host_ld
@@ -146,6 +149,9 @@ gcc_toolchain("v8_snapshot") {
   if (cxx != get_path_info(cxx, "file") && string_replace(cxx, " ", "") == cxx) {
     cxx = rebase_path(cxx, root_build_dir)
   }
+  if (ar != get_path_info(ar, "file") && string_replace(ar, " ", "") == ar) {
+    ar = rebase_path(ar, root_build_dir)
+  }
   ld = cxx
   if (cros_v8_snapshot_ld != "") {
     ld = cros_v8_snapshot_ld

build/toolchain/cros_toolchain.gni

@@ -32,6 +32,8 @@
 # cros_target_ar = "x86_64-cros-linux-gnu-gcc-ar"
 # target_cpu = "x64"
 
+import("//build/config/clang/clang.gni")
+
 declare_args() {
   # These must be specified for a board-specific build.
   cros_target_ar = "ar"
@@ -49,10 +51,10 @@ declare_args() {
   cros_target_extra_ldflags = ""
 
   # is_clang is used instead of cros_target_is_clang
-
-  cros_host_ar = "ar"
-  cros_host_cc = "gcc"
-  cros_host_cxx = "g++"
+  cros_host_ar = "${clang_base_path}/bin/llvm-ar"
+  cros_host_cc = "${clang_base_path}/bin/clang"
+  cros_host_cxx = "${clang_base_path}/bin/clang++"
+  cros_host_is_clang = true
   cros_host_ld = ""
   cros_host_nm = ""
   cros_host_readelf = ""
@@ -60,11 +62,11 @@ declare_args() {
   cros_host_extra_cppflags = ""
   cros_host_extra_cxxflags = ""
   cros_host_extra_ldflags = ""
-  cros_host_is_clang = false
 
-  cros_v8_snapshot_ar = "ar"
-  cros_v8_snapshot_cc = "gcc"
-  cros_v8_snapshot_cxx = "g++"
+  cros_v8_snapshot_ar = "${clang_base_path}/bin/llvm-ar"
+  cros_v8_snapshot_cc = "${clang_base_path}/bin/clang"
+  cros_v8_snapshot_cxx = "${clang_base_path}/bin/clang++"
+  cros_v8_snapshot_is_clang = true
   cros_v8_snapshot_ld = ""
   cros_v8_snapshot_nm = ""
   cros_v8_snapshot_readelf = ""
@@ -72,7 +74,6 @@ declare_args() {
   cros_v8_snapshot_extra_cppflags = ""
   cros_v8_snapshot_extra_cxxflags = ""
   cros_v8_snapshot_extra_ldflags = ""
-  cros_v8_snapshot_is_clang = false
 
   cros_nacl_bootstrap_extra_cflags = ""
   cros_nacl_bootstrap_extra_cppflags = ""