[build] Add MB config for mips builder

TBR=yangguo@chromium.org
NOTRY=true

Bug: chromium:616017
Change-Id: I48a29bc96e2c7ae54f4b5bbb6790db1d8bfccab4
Reviewed-on: https://chromium-review.googlesource.com/908288Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51169}

infra/mb/mb_config.pyl

@@ -157,7 +157,7 @@
           'gn_debug_simulate_arm64_no_snap',
       'V8 Linux - arm64 - sim - gc stress': 'gn_debug_simulate_arm64',
       # Mips.
-      'V8 Mips - builder': 'gyp_release_mips_no_snap_no_i18n',
+      'V8 Mips - builder': 'gn_release_mips_no_snap_no_i18n',
       'V8 Linux - mipsel - sim - builder': 'gn_release_simulate_mipsel',
       'V8 Linux - mips64el - sim - builder': 'gn_release_simulate_mips64el',
       # PPC.
@@ -535,9 +535,9 @@
       'gn', 'release_bot', 'x86', 'swarming', 'dcheck_always_on',
       'v8_enable_slow_dchecks', 'v8_verify_csa'],
 
-    # Gyp release configs for mips.
-    'gyp_release_mips_no_snap_no_i18n': [
-      'gyp', 'release', 'mips', 'crosscompile', 'no_sysroot', 'static',
+    # GN release configs for mips.
+    'gn_release_mips_no_snap_no_i18n': [
+      'gn', 'release', 'mips', 'crosscompile', 'no_sysroot', 'static',
       'v8_no_i18n', 'v8_snapshot_none'],
 
     # Gyp release configs for x64.
@@ -668,10 +668,15 @@
     },
 
     'mips': {
+      'mixins': ['mips_bundled_toolchain'],
       'gn_args': 'target_cpu="mips"',
       'gyp_defines': 'target_arch=mips',
     },
 
+    'mips_bundled_toolchain': {
+      'gn_args': 'custom_toolchain="//tools/toolchain:mips-bundled"',
+    },
+
     'msan': {
       'mixins': ['v8_enable_test_features'],
       'gn_args': ('is_msan=true msan_track_origins=2 '