The toolchains have different GN labels, but they end up writing into the same root_out_dir, since the root_out_dir is based off of just the label name and doesn't include the dir. Changing the subject from "same name" to "same root_out_dir" accordingly.

https://codereview.chromium.org/2483713003/

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/eab6e2d58dc3b71e094ff58d822980ca3473caf8

commit eab6e2d58dc3b71e094ff58d822980ca3473caf8
Author: thakis <thakis@chromium.org>
Date: Tue Nov 08 01:44:32 2016

gn: Diag on distinct toolchains with the same name.

Previously, toolchains with the same name used to silently write to the same output
directory and clobber each others generated .ninja files.  After this patch, this still
happens, but gn writes an error message while writing the final build.ninja file.

Example output:
ERROR at //build/toolchain/mac2/BUILD.gn:51:3: Duplicate toolchain.
  toolchain(target_name) {
  ^-----------------------
Two or more toolchains write to the same directory:
  //out/gn1/clang_x64/

This can be fixed by making sure that distinct toolchains have
distinct names.

See //build/toolchain/mac3/BUILD.gn:51:3: Previous toolchain.
  toolchain(target_name) {
  ^-----------------------

Depends on https://codereview.chromium.org/2485523002/

BUG= 661054 

Review-Url: https://codereview.chromium.org/2483713003
Cr-Commit-Position: refs/heads/master@{#430473}

[modify] https://crrev.com/eab6e2d58dc3b71e094ff58d822980ca3473caf8/tools/gn/ninja_build_writer.cc
[modify] https://crrev.com/eab6e2d58dc3b71e094ff58d822980ca3473caf8/tools/gn/ninja_build_writer.h