[Fuchsia] Add missing AgentImpl dependency to base test_support.

TBR: elvin
Change-Id: I7624a09957cfeb29ae157811a21f7ce3ec83b360
Reviewed-on: https://chromium-review.googlesource.com/c/1471235
Auto-Submit: Wez <wez@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#631933}

fuchsia/base/BUILD.gn

@@ -45,6 +45,7 @@ source_set("test_support") {
     "result_receiver.h",
   ]
   public_deps = [
+    ":modular",
     "//base",
     "//third_party/fuchsia-sdk/sdk:modular",
   ]