chromeos: Require lld for thinlto builds.

As far as I know, chrome-for-chromeos no longer links with gold,
so we can clean up the code a bit here.

No intended behavior change.

Bug: none
Change-Id: Idc0ee540b2ec8031cf33e483e161c118bebb4035
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2429442
Auto-Submit: Nico Weber <thakis@chromium.org>
Reviewed-by: George Burgess <gbiv@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810580}

build/config/compiler/BUILD.gn

@@ -602,8 +602,7 @@ config("compiler") {
   # TODO(pcc): Make this conditional on is_official_build rather than on gn
   # flags for specific features.
   if (!is_debug && use_thin_lto && is_a_target_toolchain) {
-    assert(use_lld || target_os == "chromeos",
-           "gold plugin only supported with ChromeOS")
+    assert(use_lld, "LTO is only supported with lld")
 
     cflags += [
       "-flto=thin",
@@ -638,7 +637,6 @@ config("compiler") {
       # Limit the parallelism to avoid too aggressive competition between
       # linker jobs. This is still suboptimal to a potential dynamic
       # resource allocation scheme, but should be good enough.
-      if (use_lld) {
       ldflags += [ "-Wl,--thinlto-jobs=" + max_jobs_per_link ]
 
       # Limit the size of the ThinLTO cache to the lesser of 10% of
@@ -650,11 +648,7 @@ config("compiler") {
             rebase_path("$root_out_dir/thinlto-cache", root_build_dir),
         "-Wl,--thinlto-cache-policy,$cache_policy",
       ]
-      } else {
-        ldflags += [ "-Wl,-plugin-opt,jobs=" + max_jobs_per_link ]
-      }
 
-      if (use_lld) {
       ldflags += [ "-Wl,--lto-O" + lto_opt_level ]
       if (thin_lto_enable_optimizations) {
         if (is_android) {
@@ -663,9 +657,6 @@ config("compiler") {
           ldflags += [ "-Wl,-mllvm,-import-instr-limit=5" ]
         }
       }
-      } else {
-        not_needed([ "lto_opt_level" ])
-      }
     }
 
     # TODO(pcc): Re-enable this flag on Android. This will require libc++ to be