Fix gn CHECK()ing when generating Chromium build files

After r531270, gn chokes while running exec_script():

[0124/122441.126364:FATAL:thread_restrictions.cc(105)] Check failed:
!g_base_sync_primitives_disallowed.Get().Get().

GN executes scripts synchronously while loading files, so blocking is necessary.
This CL adds a ScopedAllowBaseSyncPrimitives for gn to use while waiting for
exec_script() to complete.

BUG=805628
R=dpranke,thestig,fdoray

Change-Id: I7ad539a8dee0230c84d94f1c6b37755a890bfcd7
Reviewed-on: https://chromium-review.googlesource.com/884423
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532069}

base/threading/thread_restrictions.h

@@ -62,6 +62,9 @@ namespace disk_cache {
 class BackendImpl;
 class InFlightIO;
 }
+namespace functions {
+class ExecScriptScopedAllowBaseSyncPrimitives;
+}
 namespace gpu {
 class GpuChannelHost;
 }
@@ -267,6 +270,7 @@ class BASE_EXPORT ScopedAllowBaseSyncPrimitives {
                            ScopedAllowBaseSyncPrimitivesWithBlockingDisallowed);
   friend class base::GetAppOutputScopedAllowBaseSyncPrimitives;
   friend class content::SessionStorageDatabase;
+  friend class functions::ExecScriptScopedAllowBaseSyncPrimitives;
   friend class leveldb::LevelDBMojoProxy;
   friend class media::BlockingUrlProtocol;
   friend class net::MultiThreadedCertVerifierScopedAllowBaseSyncPrimitives;

tools/gn/function_exec_script.cc

@@ -7,6 +7,7 @@
 #include "base/logging.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/utf_string_conversions.h"
+#include "base/threading/thread_restrictions.h"
 #include "base/time/time.h"
 #include "build/build_config.h"
 #include "tools/gn/err.h"
@@ -115,6 +116,9 @@ Example
   exec_script("//foo/bar/myscript.py")
 )";
 
+class ExecScriptScopedAllowBaseSyncPrimitives
+    : public base::ScopedAllowBaseSyncPrimitives {};
+
 Value RunExecScript(Scope* scope,
                     const FunctionCallNode* function,
                     const std::vector<Value>& args,
@@ -221,11 +225,15 @@ Value RunExecScript(Scope* scope,
   std::string output;
   std::string stderr_output;
   int exit_code = 0;
-  if (!internal::ExecProcess(
-          cmdline, startup_dir, &output, &stderr_output, &exit_code)) {
-    *err = Err(function->function(), "Could not execute python.",
-        "I was trying to execute \"" + FilePathToUTF8(python_path) + "\".");
-    return Value();
+  {
+    ExecScriptScopedAllowBaseSyncPrimitives allow_base_sync_primitives;
+    if (!internal::ExecProcess(cmdline, startup_dir, &output, &stderr_output,
+                               &exit_code)) {
+      *err = Err(
+          function->function(), "Could not execute python.",
+          "I was trying to execute \"" + FilePathToUTF8(python_path) + "\".");
+      return Value();
+    }
   }
   if (g_scheduler->verbose_logging()) {
     g_scheduler->Log("Pythoning", script_source.value() + " took " +