Add precompiled headers to GN build for large targets.

content/test/BUILD.gn

Index: content/test/BUILD.gn
diff --git a/content/test/BUILD.gn b/content/test/BUILD.gn
index 0dbffbdb0022a76a505f145e135c6838fb53a41f..23c600cc56b9d1b066d4e1c55bdf940211b591e4 100644
--- a/content/test/BUILD.gn
+++ b/content/test/BUILD.gn
@@ -22,6 +22,7 @@ content_tests_gypi_values =
 # GYP version //content/content_tests.gypi:test_support_content
 source_set("test_support") {
   testonly = true
+  configs += [ "//build/config:precompiled_headers" ]
   public_deps = [
     "//content/public/app:both",
     "//content/public/browser",
@@ -268,7 +269,10 @@ if (!is_mac) {
 
     defines = [ "HAS_OUT_OF_PROC_TEST_RUNNER" ]
 
-    configs += [ "//build/config/compiler:no_size_t_to_int_warning" ]
+    configs += [
+      "//build/config:precompiled_headers",
+      "//build/config/compiler:no_size_t_to_int_warning",
+    ]
 
     deps = [
       ":browsertest_support",
@@ -427,6 +431,9 @@ test("content_unittests") {
   sources = rebase_path(content_tests_gypi_values.content_unittests_sources,
                         ".",
                         "//content")
+
+  configs += [ "//build/config:precompiled_headers" ]
+
   deps = [
     ":test_support",
     "//base/allocator",