Add a mojo bridge for PersistentPrefStore.

content/browser/browser_main_runner.cc

Index: content/browser/browser_main_runner.cc
diff --git a/content/browser/browser_main_runner.cc b/content/browser/browser_main_runner.cc
index 4b3c19c4aa7b7c90b8615594eaafb386078783ad..736a23af01dd76fda4268c02222f8596e9379119 100644
--- a/content/browser/browser_main_runner.cc
+++ b/content/browser/browser_main_runner.cc
@@ -27,6 +27,7 @@
 #include "content/public/browser/tracing_controller.h"
 #include "content/public/common/content_switches.h"
 #include "content/public/common/main_function_params.h"
+#include "mojo/public/cpp/bindings/sync_call_restrictions.h"
 #include "third_party/skia/include/core/SkGraphics.h"
 #include "ui/base/ime/input_method_initializer.h"
 
@@ -60,6 +61,7 @@ class BrowserMainRunnerImpl : public BrowserMainRunner {
   }
 
   int Initialize(const MainFunctionParams& parameters) override {
+    mojo::SyncCallRestrictions::ScopedAllowSyncCall allow_sync_calls;
     SCOPED_UMA_HISTOGRAM_LONG_TIMER(
         "Startup.BrowserMainRunnerImplInitializeLongTime");