build: move chromeos' import-instr-limits into chrome

These existed in the ebuild before for no particular reason. Having them
sitting in Chrome seems more theoretically good, and fixes a small bug
with some of our simplechrome builders (they weren't picking up the =20
or =30 limits properly, so they were doing =100...)

Bug: 937821
Change-Id: I3f89354d377de2e4db7c1b11b644b07867d3b66c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2446818
Commit-Queue: George Burgess <gbiv@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813446}

build/config/compiler/BUILD.gn

@@ -656,10 +656,25 @@ config("compiler") {
 
       ldflags += [ "-Wl,--lto-O" + lto_opt_level ]
       if (thin_lto_enable_optimizations) {
+        # TODO(gbiv): We ideally shouldn't need to specify this; ThinLTO
+        # should be able to better manage binary size increases on its own.
+        import_instr_limit = -1
         if (is_android) {
-          # TODO(gbiv): We ideally shouldn't need to specify this; ThinLTO
-          # should be able to better manage binary size increases on its own.
-          ldflags += [ "-Wl,-mllvm,-import-instr-limit=5" ]
+          import_instr_limit = 5
+        } else if (is_chromeos) {
+          # Not much performance difference was noted between the default (100)
+          # and these. ARM was originally set lower than x86 to keep the size
+          # bloat of ThinLTO to <10%, but that's potentially no longer true.
+          # FIXME(inglorion): maybe tune these?
+          if (target_cpu == "arm" || target_cpu == "arm64") {
+            import_instr_limit = 20
+          } else {
+            import_instr_limit = 30
+          }
+        }
+
+        if (import_instr_limit != -1) {
+          ldflags += [ "-Wl,-mllvm,-import-instr-limit=$import_instr_limit" ]
         }
       }
     }