Add compiler and linker flags for MIPS.

build/config/mips.gni

+# Copyright 2015 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+# MIPS arch variant.
+if (cpu_arch == "mipsel") {
+  declare_args() {

[brettw, 2015/01/12 07:27:02]

Will anybody actually need to set the arch variant manually from outside the
build? Most of the things we have parameterized this way are never used. I've
been taking the approach of just declaring regular variables (no declare_args)
and if somebody needs to set it they can come along and add declare_args.

[wtc, 2015/01/12 22:53:49]

Henrik: does the WebRTC team need to set mips_arch_variant manually from outside
the build?

[petarj, 2015/01/14 15:08:55]

We actually use this often.
Being able to build Chromium for Linux mips32r1 and mips32r2 without
changing the source code is very convenient.

+    mips_arch_variant = "r1"

[Ben Chan, 2015/01/20 23:36:34]

this definition differs from the one in webrtc.gni, and probably causes a
redefinition error.

[wtc, 2015/01/21 01:58:50]

Yes. This Chromium CL requires a companion WebRTC CL: 
https://webrtc-codereview.appspot.com/41399004/

Re: "mips32r1" vs. "r1": I decided to go with "r1" because that is
the form used in the GYP build system (build/common.gypi).

+  }
+}
+if (cpu_arch == "mips64el") {

[brettw, 2015/01/20 23:34:26]

Can you make this an "else if"

[wtc, 2015/01/21 01:58:50]

Done.

+  if (is_android) {
+    declare_args() {
+      mips_arch_variant = "r6"
+    }
+  } else {
+    declare_args() {
+      mips_arch_variant = "r2"
+    }
+  }
+}