Enable ThinLTO for POSIX LTO by default on Linux, take 2.

build/toolchain/toolchain.gni

 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # Toolchain-related configuration that may be needed outside the context of the
 # toolchain() rules themselves.
 
 import("//build_overrides/build.gni")
 import("//build/config/chrome_build.gni")
 
 declare_args() {
   # Enable Link Time Optimization in optimized builds (output programs run
   # faster, but linking is up to 5-20x slower).
   # Note: use target_os == "linux" rather than is_linux so that it does not
   # apply to host_toolchain when target_os="android".
   allow_posix_link_time_opt =
       is_clang && target_os == "linux" && !is_chromeos && target_cpu == "x64" &&
       is_official_build
+}
 
-  # If used with allow_posix_link_time_opt, it enables the experimental support
-  # of ThinLTO that links 3x-10x faster but (as of now) does not have all the
-  # important optimizations such us devirtualization implemented. See also
+declare_args() {
+  # If used with allow_posix_link_time_opt, it enables support for ThinLTO,
+  # which links 3x-10x faster than full LTO. See also
   # http://blog.llvm.org/2016/06/thinlto-scalable-and-incremental-lto.html
-  use_thin_lto = false
+  use_thin_lto = allow_posix_link_time_opt && target_os == "linux" &&
+                 !is_chromeos && target_cpu == "x64"
 
   # If this is set to true, or if LLVM_FORCE_HEAD_REVISION is set to 1
   # in the environment, we use the revision in the llvm repo to determine
   # the CLANG_REVISION to use, instead of the version hard-coded into
   # //tools/clang/scripts/update.py. This should only be used in
   # conjunction with setting LLVM_FORCE_HEAD_REVISION in the
   # environment when `gclient runhooks` is run as well.
   llvm_force_head_revision = false
 
   # Compile with Xcode version of clang instead of hermetic version shipped
@@ skipping 85 matching lines @@
   _tool_wrapper_path =
       rebase_path("//build/toolchain/win/tool_wrapper.py", root_build_dir)
 
   stamp_command = "$python_path $_tool_wrapper_path stamp {{output}}"
   copy_command =
       "$python_path $_tool_wrapper_path recursive-mirror {{source}} {{output}}"
 } else {
   stamp_command = "touch {{output}}"
   copy_command = "ln -f {{source}} {{output}} 2>/dev/null || (rm -rf {{output}} && cp -af {{source}} {{output}})"
 }