Enforce that the allocator shim be enabled for partitionalloc

If you disable the allocator shim while partition alloc is enabled then
you will get cryptic linker errors. This change detects these
incompatible settings earlier to give a clearer error message.

Change-Id: Ifb03d8fccd7c12e89ef62222b34ba3d8dfdd704b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2594122Reviewed-by: Benoit L <lizeb@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837613}

base/allocator/BUILD.gn

@@ -289,6 +289,8 @@ buildflag_header("buildflags") {
   header = "buildflags.h"
   _use_partition_alloc = use_allocator == "partition"
   _use_tcmalloc = use_allocator == "tcmalloc"
+  assert(use_allocator_shim || !_use_partition_alloc,
+         "Partition alloc requires the allocator shim")
 
   flags = [
     "USE_ALLOCATOR_SHIM=$use_allocator_shim",