drop warnings and -Werror from is_official_builds

People using is_official_build don't really want to see warnings.
They're for devs, not users.

The somewhat odd update to gn/BUILDCONFIG.gn keeps command
line flag precedence (later == more important) unchanged.

Change-Id: I1a04a35f066b7408021d474535f0dbf4928e21d3
Reviewed-on: https://skia-review.googlesource.com/151380
Commit-Queue: Brian Salomon <bsalomon@google.com>
Auto-Submit: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
diff --git a/gn/BUILD.gn b/gn/BUILD.gn
index d6fd5c2..3dcdbed 100644
--- a/gn/BUILD.gn
+++ b/gn/BUILD.gn
@@ -94,7 +94,6 @@
     cflags += [
       "-fstrict-aliasing",
       "-fPIC",
-      "-Werror",
     ]
     cflags_cc += [ "-std=c++14" ]
 
@@ -308,6 +307,7 @@
     ]
   } else {
     cflags += [
+      "-Werror",
       "-Wall",
       "-Wextra",
       "-Winit-self",