Fix headless component build on Windows

In recent change crrev.com/c/1263566, HeadlessContentUtilityClient
will be exported in component build on Windows. As usual, it should
be exported in headless.dll. However, library headless_shell_lib
also includes this class which results compilation errors.

This CL excludes HeadlessContentUtilityClient from
headless_shell_lib in component build to resolve the breakage.

BUG=894835

Change-Id: I44853a4c7a0701b9ea29cc16b18e9c3b358cb148
Reviewed-on: https://chromium-review.googlesource.com/c/1277883
Commit-Queue: Wei Li <weili@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599324}

headless/BUILD.gn

@@ -852,8 +852,6 @@ jumbo_static_library("headless_shell_lib") {
   sources = [
     "app/headless_shell.cc",
     "app/headless_shell.h",
-    "lib/utility/headless_content_utility_client.cc",
-    "lib/utility/headless_content_utility_client.h",
     "public/headless_shell.h",
   ]
 
@@ -861,6 +859,8 @@ jumbo_static_library("headless_shell_lib") {
     sources += [
       "lib/browser/headless_content_browser_client.cc",
       "lib/browser/headless_content_browser_client.h",
+      "lib/utility/headless_content_utility_client.cc",
+      "lib/utility/headless_content_utility_client.h",
     ]
   }