Upstream helper script to build Chromium on iOS and update instructions.

ios/build/tools/setup-gn.config

+[goma]
+# Controls whether goma is enabled or not. If you generally use goma but
+# want to disable goma for a single build, consider using the environment
+# variable GOMA_DISABLED.
+enabled = False
+install = "$GOMA_DIR"
+
+[xcode]
+# Controls settings for the generated Xcode project. If jobs is non-zero
+# it will be passed to the ninja invocation in Xcode project.
+jobs = 0
+
+[build]
+# Controls the build output. The only supported values are "64-bit", "32-bit"
+# and "multi" (for a fat binary supporting both "32-bit" and "64-bit" cpus).
+arch = "64-bit"
+
+[gn_args]
+# Values in that section will be copied verbatim in the generated args.gn file.
+target_os = "ios"
+
+[filters]
+# List of target files to pass to --filters argument of gn gen when generating
+# the Xcode project. By default, list all targets from ios/ and ios_internal/
+# and the targets corresponding to the unit tests run on the bots.
+filter_0 = "//base:base_unittests"
+filter_1 = "//components:components_unittests"
+filter_2 = "//crypto:crypto_unittests"
+filter_3 = "//google_apis:google_apis_unittests"
+filter_4 = "//ios/*"
+filter_5 = "//ios_internal/*"
+filter_6 = "//net:net_unittests"
+filter_7 = "//skia:skia_unittests"
+filter_8 = "//sql:sql_unittests"
+filter_9 = "//ui/base:ui_base_unittests"
+filter_a = "//ui/gfx:gfx_unittests"
+filter_b = "//url:url_unittests"