Increase -j for android-marshmallow-arm64-rel builder

This is for more than 30 mins compile steps like below builds.

2978s https://ci.chromium.org/b/8939325304285978864
2782s https://ci.chromium.org/b/8939363044758981744
2702s https://ci.chromium.org/b/8939330795897045200
2595s https://ci.chromium.org/b/8939375915479218880
2548s https://ci.chromium.org/b/8939363618226431392

Bug: 804251
Change-Id: I0b2526a12712079080a38ca72e82f9a9a32d524a
Reviewed-on: https://chromium-review.googlesource.com/1160139Reviewed-by: Nodir Turakulov <nodir@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580411}

infra/config/global/cr-buildbucket.cfg

@@ -2351,7 +2351,7 @@ buckets {
     }
     builders {
       mixins: "android-try"
-      mixins: "goma-j150"
+      mixins: "goma-j300"
       name: "android-marshmallow-arm64-rel"
       dimensions: "os:Ubuntu-14.04"
     }