Reland of Enable LTO on Linux for the official builds.

build/toolchain/toolchain.gni

Index: build/toolchain/toolchain.gni
diff --git a/build/toolchain/toolchain.gni b/build/toolchain/toolchain.gni
index 4ab0270f902a7b8dfd9b3114b0281e787d70265e..9f564794816655edd06d110aeb2ce14390f037bb 100644
--- a/build/toolchain/toolchain.gni
+++ b/build/toolchain/toolchain.gni
@@ -5,12 +5,16 @@
 # Toolchain-related configuration that may be needed outside the context of the
 # toolchain() rules themselves.
 
+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).
-  #
-  # TODO(pcc): Remove this flag if/when LTO is enabled in official builds.
-  allow_posix_link_time_opt = false
+  # 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 =
+      target_os == "linux" && !is_chromeos && target_cpu == "x64" &&
+      is_chrome_branded && is_official_build

[Dirk Pranke, 2016/05/26 18:26:29]

nit: is_chromeos will never be true of target_os == "linux", so you don't need
that clause.

 
   # 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