Add GN Build file for extensions_browser target.

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 4def35517de72e4be5ee4cc63350c8802c53d8d2..c40411b923dcfb9af773b4f948f2814f4ea314b0 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -37,6 +37,7 @@ group("root") {
     "//device/bluetooth",
     "//device/nfc",
     "//device/usb",
+    "//extensions/browser",
     "//extensions/common",
     "//extensions/common/api",
     "//gin",
@@ -174,6 +175,7 @@ group("root") {
       "//content",
       "//content/shell:content_shell",  # Blocked on content.
       "//content/test:test_support",  # Blocked on content.
+      "//extensions/browser",
       "//extensions/common",
       "//extensions/common/api",
       "//pdf",  # Not compiled on Android in GYP yet, either.