Add dependency on safe_browsing:csd_proto to c/b/extensions.

c/b/extensions/api/downloads/downloads_api.h indirectly includes the
generated csd.pb.h file, so it needs an explicit dependency on the
target that produces it.

BUG=856920
TBR=jialiul@chromium.org

Change-Id: I8a1821b05b92c71fa89bb706d6e9b21abf93bc93
Reviewed-on: https://chromium-review.googlesource.com/1116539
Commit-Queue: Greg Thompson <grt@chromium.org>
Reviewed-by: Jialiu Lin <jialiul@chromium.org>
Reviewed-by: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570692}

chrome/browser/extensions/BUILD.gn

@@ -787,6 +787,7 @@ jumbo_static_library("extensions") {
   # browser, then we can clean up these dependencies.
   public_deps = [
     "//chrome/common/extensions/api",
+    "//components/safe_browsing:csd_proto",
     "//components/safe_browsing/db:util",
     "//content/public/browser",
   ]