Don't default use_thin_lto to true in official ARC builds.

This will cause it to be false in ARC-bits of chrome-branded linux-chromeos builds.

This should reduce the maintenance burden of Chrome's clang since there's
a non-zero cost of ensuring the official linux-chromeos build config builds
with thin lto.

Disabling it should be safe since linux-chromeos is a developer-only build,
and actual builds of Chrome OS explicitly set it:
https://chromium.googlesource.com/chromiumos/overlays/chromiumos-overlay/+/3c6956bccc6e77a38395f35919d43c502f6c5cdf/chromeos-base/chromeos-chrome/chromeos-chrome-9999.ebuild#271

Bug: 1033839
Change-Id: I8d44482a6cea956afba808480aeed69c64e5c6e3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1967602
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#727093}

build/config/compiler/compiler.gni

@@ -56,7 +56,10 @@ declare_args() {
 
   # 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 = is_cfi || (is_android && is_official_build)
+  # Use it by default on official-optimized android builds, but not ARC since
+  # it's been seen to not play nicely with Chrome's clang.
+  use_thin_lto =
+      is_cfi || (is_android && target_os != "chromeos" && is_official_build)
 
   # Limit the number of jobs (threads/processes) the linker is allowed
   # to use (for linkers that support this).