linux_aura: run IconLoader::ReadIcon on the UI thread.

chrome/browser/icon_loader_mac.mm

Index: chrome/browser/icon_loader_mac.mm
diff --git a/chrome/browser/icon_loader_mac.mm b/chrome/browser/icon_loader_mac.mm
index be2eecf5bed5d9499a8cc1f09a0a2bdf6aa4f426..0757d1f005ef1ca8cd3a36467382d902b24a15c4 100644
--- a/chrome/browser/icon_loader_mac.mm
+++ b/chrome/browser/icon_loader_mac.mm
@@ -20,10 +20,16 @@ IconGroupID IconLoader::ReadGroupIDFromFilepath(
   return filepath.Extension();
 }
 
+// static
 bool IconLoader::IsIconMutableFromFilepath(const base::FilePath&) {
   return false;
 }
 
+// static
+content::BrowserThread::ID IconLoader::ReadIconThreadID() {
+  return content::BrowserThread::FILE;
+}
+
 void IconLoader::ReadIcon() {
   NSString* group = base::SysUTF8ToNSString(group_);
   NSWorkspace* workspace = [NSWorkspace sharedWorkspace];