Add comments about what should and should not depend on 'allocator'

base/allocator/allocator.gyp

Index: base/allocator/allocator.gyp
diff --git a/base/allocator/allocator.gyp b/base/allocator/allocator.gyp
index 821289f530dd90f5bfd62a962862ed83bc8c0b06..f5bda7370c1b5decbe6fa05eb5a77f51a5fc9543 100644
--- a/base/allocator/allocator.gyp
+++ b/base/allocator/allocator.gyp
@@ -9,6 +9,9 @@
     'use_vtable_verify%': 0,
   },
   'targets': [
+    # Only executables and not libraries should depend on the
+    # allocator target; only the application (the final executable)
+    # knows what allocator makes sense.
     {
       'target_name': 'allocator',
       'type': 'static_library',