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..1eae20159efd302d31de93d7d4df4cb42eabb67f 100644
--- a/build/toolchain/toolchain.gni
+++ b/build/toolchain/toolchain.gni
@@ -5,12 +5,14 @@
 # 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
+  allow_posix_link_time_opt = is_linux && !is_chromeos && target_cpu == "x64" &&
+                              is_chrome_branded && 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