Add back PPAPI headers to the source list.

A couple of them got removed in https://crrev.com/694112.

Change-Id: If299e30f0063a7ed20b6091aa2cd0919525d868c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1790584Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694531}

content/test/BUILD.gn

@@ -482,7 +482,9 @@ jumbo_static_library("test_support") {
       "../public/test/fake_pepper_plugin_instance.cc",
       "../public/test/fake_pepper_plugin_instance.h",
       "../public/test/ppapi_test_utils.cc",
+      "../public/test/ppapi_test_utils.h",
       "ppapi_unittest.cc",
+      "ppapi_unittest.h",
     ]
     deps += [
       "//content/ppapi_plugin",