Use depfiles for mojom generation.

mojo/public/tools/bindings/mojom.gni

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 declare_args() {
   # Indicates whether typemapping should be supported in this build
   # configuration. This may be disabled when building external projects which
   # depend on //mojo but which do not need/want all of the Chromium tree
   # dependencies that come with typemapping.
   #
@@ skipping 241 matching lines @@
       deps = [
         "//mojo/public/tools/bindings:precompile_templates",
       ]
       outputs = generator_shared_cpp_outputs
       args = common_generator_args
       args += [
         "--generate_non_variant_code",
         "-g",
         "c++",
       ]
+      depfile = "{{source_gen_dir}}/${generator_shared_target_name}_{{source_name_part}}.d"
+      args += [
+        "--depfile",
+        depfile,
+        "--depfile_target",
+        "{{source_gen_dir}}/{{source_name_part}}.mojom-shared-internal.h",
+      ]
     }
   }
 
   shared_cpp_sources_suffix = "shared_cpp_sources"
   shared_cpp_sources_target_name = "${target_name}_${shared_cpp_sources_suffix}"
   source_set(shared_cpp_sources_target_name) {
     if (defined(invoker.testonly)) {
       testonly = invoker.testonly
     }
     deps = []
@@ skipping 109 matching lines @@
             "c++,javascript,java",
           ]
         }
 
         if (defined(bindings_configuration.variant)) {
           args += [
             "--variant",
             bindings_configuration.variant,
           ]
         }
+        depfile =
+            "{{source_gen_dir}}/${generator_target_name}_{{source_name_part}}.d"
+        args += [
+          "--depfile",
+          depfile,
+          "--depfile_target",
+          "{{source_gen_dir}}/{{source_name_part}}.mojom${variant_dash_suffix}.cc",
+        ]
 
         args += [
           "--typemap",
           rebase_path(type_mappings_path, root_build_dir),
         ]
 
         if (defined(bindings_configuration.for_blink) &&
             bindings_configuration.for_blink) {
           args += [ "--for_blink" ]
           if (defined(invoker.export_class_attribute_blink)) {
@@ skipping 224 matching lines @@
           full_name = get_label_info(d, "label_no_toolchain")
           deps += [ "${full_name}_java" ]
         }
 
         srcjar_deps = [ ":$java_srcjar_target_name" ]
         run_findbugs_override = false
       }
     }
   }
 }