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

build/toolchain/toolchain.gni

Index: build/toolchain/toolchain.gni
diff --git a/build/toolchain/toolchain.gni b/build/toolchain/toolchain.gni
index 70e0fa96037e7cd5e5c175bcc895482aeb0d91d3..63ecdbd89a3d9c4ac7dcff88ec41f42ba1976500 100644
--- a/build/toolchain/toolchain.gni
+++ b/build/toolchain/toolchain.gni
@@ -16,12 +16,14 @@ declare_args() {
   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