Make MB aware of iOS bot configs and get iOS working.

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index d1791b096917fd38cc9a71d4fbd0aa028737d26f..05fb68827364828b04c7b047753f7d53fccb8a12 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -25,7 +25,7 @@ declare_args() {
   root_extra_deps = []
 }
 
-# This file defines the following four main targets:
+# This file defines the following five main targets:
 #
 # "both_gn_and_gyp" should list every root target (target that nothing else
 # depends on) built by GN that is also built in the GYP build.
@@ -33,19 +33,28 @@ declare_args() {
 # "gn_all" should (transitively) cause everything to be built; if you run
 # 'ninja gn_all' and then 'ninja all', the second build should do no work.
 #
-# "gn_only" should list every root target that is *not* intended to be built
-# in a GYP build. Because GN has different rules for deciding what an 'all'
-# build is, this may end up including targets that are actually defined in a
-# GYP build but not dependencies of GYP's "all" (and so not actually built).
+# "gn_only" should list every root target that is *not* intended to be built in
+# a GYP build. Because GN has different rules for deciding what an 'all' build
+# is, this may end up including targets that are actually defined in a GYP
+# build but not dependencies of GYP's "all" (and so not actually built).
 #
 # "gn_visibility": targets that are normally not visible to top-level targets,
-# but are built anyway by "all". Since we don't want any such targets, we
-# have this placeholder to make sure hidden targets that aren't otherwise
-# depended on yet are accounted for.
+# but are built anyway by "all". Since we don't want any such targets, we have
+# this placeholder to make sure hidden targets that aren't otherwise depended
+# on yet are accounted for.
+#
+# "All" is an alias for "gn_all". It exists for bot compatibility w/ GYP for
+# the iOS bots, but should not be generally used during the GYP->GN migration.
+# We cannot guarantee that GN's "All" builds the same set of targets as GYP's
+# "All" does, because GYP's "All" supports wildcards.
+#
+# Lastly, none of these targets are guaranteed to be the same as what ninja
+# will build with "all". For more on how "all" works and the differences in how
+# GYP and GN determine "all", see crbug.com/503241.
 #
 # TODO(GYP): crbug.com/481694. Make sure that the above is true and there are
-# scripts run on the bots that enforce this. Once the GYP migration is over,
-# we can collapse all of these targets as desired.
+# scripts run on the bots that enforce this. Once the GYP migration is over, we
+# can collapse all of these targets as desired.
 
 group("gn_all") {
   testonly = true
@@ -57,6 +66,16 @@ group("gn_all") {
   ]
 }
 
+# TODO(GYP): This target exists for compatibility with GYP, specifically
+# for the iOS bots. We should update the iOS bots to not pass 'All'.
+group("All") {
+  testonly = true
+
+  deps = [
+    ":gn_all",
+  ]
+}
+
 # The "both_gn_and_gyp" target should reflect every target that is built
 # in both the GN and GYP builds, and ideally it should match the
 # "both_gn_and_gyp" target in build/gn_migration.gypi line-for-line.