Enable ThinLTO and LLD for POSIX LTO by default on Linux.

build/toolchain/toolchain.gni

Index: build/toolchain/toolchain.gni
diff --git a/build/toolchain/toolchain.gni b/build/toolchain/toolchain.gni
index 6eb3c9080393b1b80c79650257bcdd0b5dd745d9..7da351d6c9a5f808bfa431b519e28f8f88a085c1 100644
--- a/build/toolchain/toolchain.gni
+++ b/build/toolchain/toolchain.gni
@@ -16,18 +16,6 @@ declare_args() {
   allow_posix_link_time_opt = target_os == "linux" && !is_chromeos &&
                               target_cpu == "x64" && is_official_build
 
-  # Set to true to use lld, the LLVM linker. This flag may be used on Windows
-  # with the shipped LLVM toolchain, or on Linux with a self-built top-of-tree
-  # LLVM toolchain (see llvm_force_head_revision in
-  # 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
@@ -52,6 +40,18 @@ declare_args() {
     # Clang compiler version. Clang files are placed at version-dependent paths.
     clang_version = "5.0.0"
   }
+
+  # Set to true to use lld, the LLVM linker. This flag may be used on Windows
+  # or Linux.
+  use_lld = (is_win && host_os != "win") ||
+            (allow_posix_link_time_opt && target_os == "linux" &&
+             !is_chromeos && target_cpu == "x64")
+
+  # If used with allow_posix_link_time_opt, it enables the support of ThinLTO
+  # that links 3x-10x faster. See also
+  # http://blog.llvm.org/2016/06/thinlto-scalable-and-incremental-lto.html
+  use_thin_lto = allow_posix_link_time_opt && target_os == "linux" &&
+                 !is_chromeos && target_cpu == "x64"
 }
 
 # Check target_os here instead of is_ios as this file is loaded for secondary