Add use_thin_lto switch to start evaluating ThinLTO.

build/toolchain/toolchain.gni

Index: build/toolchain/toolchain.gni
diff --git a/build/toolchain/toolchain.gni b/build/toolchain/toolchain.gni
index b7bfaff7897a11fdb73d25b4bcf882a6f2914218..6251fe05947f988ceff0163112de1da0f7c751a6 100644
--- a/build/toolchain/toolchain.gni
+++ b/build/toolchain/toolchain.gni
@@ -22,6 +22,12 @@ declare_args() {
   # build/config/compiler/BUILD.gn).
   use_lld = is_win && host_os != "win"
 
+  # 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
+  # http://blog.llvm.org/2016/06/thinlto-scalable-and-incremental-lto.html
+  use_thin_lto = false
+
   # 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