Make the new tcmalloc the default for Chrome OS

The new tcmalloc has been enabled on three Chrome OS boards since R72,
and on ten additional boards accounting for over 50% of users in the
wild since R75. It has also been the default on Linux for over one month.

BUG=chromium:724399,b:70905156

Change-Id: Id1f49f66a963c9a327edeee3e85d0d128c9de78c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1630233
Commit-Queue: Gabriel Marin <gmx@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664303}

build/config/allocator.gni

@@ -34,7 +34,7 @@ declare_args() {
   use_partition_alloc = !is_ios
 
   # Use the new tcmalloc. It's relevant only when use_allocator == "tcmalloc".
-  use_new_tcmalloc = is_linux && !is_chromeos && !is_chromecast
+  use_new_tcmalloc = !is_chromecast
 }
 
 if (is_nacl) {