[tcmalloc] Provide low memory footprint build option for tcmalloc.

Bug: 856704
Change-Id: I48b28d389aa602718c5dae2afed98dc19322dd24
Reviewed-on: https://chromium-review.googlesource.com/1115356Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Jason Roberts <jasonroberts@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570802}

base/allocator/BUILD.gn

@@ -11,6 +11,9 @@ declare_args() {
   # e.g. for profiling (it's more rare to profile Debug builds,
   # but people sometimes need to do that).
   enable_debugallocation = is_debug
+
+  # Provide a way to build tcmalloc with a low memory footprint.
+  use_tcmalloc_small_but_slow = false
 }
 
 # This "allocator" meta-target will forward to the default allocator according
@@ -36,6 +39,9 @@ config("tcmalloc_flags") {
   if (use_allocator_shim) {
     defines += [ "TCMALLOC_DONT_REPLACE_SYSTEM_ALLOC" ]
   }
+  if (use_tcmalloc_small_but_slow) {
+    defines += [ "TCMALLOC_SMALL_BUT_SLOW" ]
+  }
   if (is_clang) {
     cflags = [
       # tcmalloc initializes some fields in the wrong order.