Fix missing dependency of //extensions:extensions_renderer_resources.

This resolves "ninja explain" failures after:
https://chromium.googlesource.com/chromium/src/+/b7917313d7e4f0d8c83ebc93ef6d6db5e79ad643

e.g.
https://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_rel_ng/builds/297782
ninja explain: restat of output gen/extensions/extensions_renderer_resources_stamp.d.stamp older than most recent input /b/c/b/linux/src/out/Release/gen/mojo/common/common_custom_types.mojom.js (0 vs 1473884135)

BUG=629375
TBR=btolsch@chromium.org
NOTRY=true

Review-Url: https://codereview.chromium.org/2338993003
Cr-Commit-Position: refs/heads/master@{#418672}

extensions/BUILD.gn

@@ -54,6 +54,7 @@ grit("extensions_renderer_resources") {
     "//device/serial:serial_mojo__generator",
     "//extensions/common:mojo__generator",
     "//extensions/common/api:mojom__generator",
+    "//mojo/common:common_custom_types__generator",
   ]
 }