Add yasm to the GN build

content/public/browser/BUILD.gn

Index: content/public/browser/BUILD.gn
diff --git a/content/public/browser/BUILD.gn b/content/public/browser/BUILD.gn
index 98bd536763abe93581b15d36e3ba8213e6daf353..ee41b2003de681ae864524a39420edff13e86290 100644
--- a/content/public/browser/BUILD.gn
+++ b/content/public/browser/BUILD.gn
@@ -5,8 +5,22 @@
 import("//content/browser/browser.gni")
 
 source_set("browser") {
-  sources = rebase_path(content_browser_gypi_values.public_browser_sources,
-                        ".", "//content")
+  if (is_ios) {
+    # iOS doesn't get the normal file list and only takes these whitelisted
+    # files.
+    sources = [
+      "browser_main_parts.cc",
+      "content_browser_client.cc",
+      "favicon_status.cc",
+      "navigation_details.cc",
+      "notification_registrar.cc",
+      "page_navigator.cc",
+      "web_ui_controller.cc",
+    ]
+  } else {
+    sources = rebase_path(content_browser_gypi_values.public_browser_sources,
+                          ".", "//content")
+  }
 
   configs += [ "//content:content_implementation" ]