.idl files aren't allowed in GN sources.

GN change disallows sources that aren't a source, header, or object file.
Changed chromium_src/BUILD.gn to use `midl` action for the
google_update_idl.idl file and then list that action as a dep for
`google_update` source_set.

    GN change:

    https://gn.googlesource.com/gn/+/8730b0feb6b991fa47368566501ab9ccfb453c92

    commit  8730b0feb6b991fa47368566501ab9ccfb453c92
    author  Tom Anderson <thomasanderson@chromium.org>      Thu May 16 11:00:40 2019 -0700
    committer       Commit Bot <commit-bot@chromium.org>    Thu May 16 21:36:50 2019 +0000

    Disallow non-buildable sources in binary targets

    When adding a file that's not a source, header, or object file to a source_set,
    loadable_module, shared_library, executable, or static_library, gn will now
    generate an error like the following:

    ERROR at //third_party/protobuf/proto_library.gni:369:15: Only source, header,
    and object files belong in the sources of a
    source_set. //out/Test/pyproto/google_apis/gcm/protocol/mcs_pb2.py is not one of
    the valid types.
        sources = get_target_outputs(":$action_name")
                  ^---------------------------------
    See //google_apis/gcm/BUILD.gn:78:1: whence it was called.
    proto_library("proto") {
    ^-----------------------
    See //BUILD.gn:89:7: which caused the file to be included.
          "//google_apis/gcm:gcm_unit_tests",
          ^---------------------------------

    BUG=77

chromium_src/BUILD.gn

@@ -1,10 +1,17 @@
 if (is_win) {
-  source_set("google_update") {
+  import("//build/toolchain/win/midl.gni")
+  midl("google_update_idl") {
     # This idl file is generated by Omaha.
     sources = [
       "google_update/google_update_idl.idl",
     ]
   }
+
+  source_set("google_update") {
+    deps = [
+      ":google_update_idl",
+    ]
+  }
 }
 
 source_set("browser") {