[CFI] Enable cfi-icall for official Linux x64 builds

The measured size impact is 1.5% and perf impact is ~1%.

Bug: 701919
Change-Id: I541242711bc85cc124ff3a2680171577e75475bd
Reviewed-on: https://chromium-review.googlesource.com/1033918
Commit-Queue: Peter Collingbourne <pcc@chromium.org>
Reviewed-by: Peter Collingbourne <pcc@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#554828}

build/config/sanitizers/sanitizers.gni

@@ -61,7 +61,8 @@ declare_args() {
   # Enable checks for indirect function calls via a function pointer.
   # TODO(pcc): remove this when we're ready to add these checks by default.
   # https://crbug.com/701919
-  use_cfi_icall = false
+  use_cfi_icall = target_os == "linux" && !is_chromeos && target_cpu == "x64" &&
+                  is_official_build
 
   # Print detailed diagnostics when Control Flow Integrity detects a violation.
   use_cfi_diag = false