Add a GN flag for the Windows multi-dll build.

The first pass of moving the other Chrome-specific build flags out of the root build config file and into the new one. This requires both ffmpeg and Blink changes to finish landing.

Originally landed as https://codereview.chromium.org/1120803003/
[email protected]
[email protected] (widevine)
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/1123433002

Cr-Commit-Position: refs/heads/master@{#327998}
diff --git a/chrome/browser/BUILD.gn b/chrome/browser/BUILD.gn
index 9bb4270..fc384b76 100644
--- a/chrome/browser/BUILD.gn
+++ b/chrome/browser/BUILD.gn
@@ -2,6 +2,7 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//build/config/chrome_build.gni")
 import("//build/config/crypto.gni")
 import("//build/config/features.gni")
 import("//build/config/ui.gni")