Reland "Make the new tcmalloc the default for Chrome OS"

This is a reland of fa4a217a

Original change's description:
> 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}

Bug: chromium:724399, b:70905156
Change-Id: I3f377fc3094c7231b4282ccdd3d56cb4dd3d463c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1636615Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Gabriel Marin <gmx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664632}

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) {