Enable the multi_dll build of Chrome in GN on Windows

CQ_EXTRA_TRYBOTS=tryserver.chromium.linux:android_chromium_gn_compile_dbg,android_chromium_gn_compile_rel;tryserver.chromium.win:win8_chromium_gn_rel,win8_chromium_gn_dbg;tryserver.chromium.mac:mac_chromium_gn_rel,mac_chromium_gn_dbg

Review URL: https://codereview.chromium.org/1119193002

Cr-Commit-Position: refs/heads/master@{#328055}

build/config/chrome_build.gni

@@ -18,6 +18,5 @@ declare_args() {
   # Break chrome.dll into multple pieces based on process type. Only available
   # on Windows.
   # TODO(brettw) make this work. When it does, the declaration should be:
-  #is_multi_dll_chrome = is_win && !is_component_build
-  is_multi_dll_chrome = false
+  is_multi_dll_chrome = is_win && !is_component_build
 }