GCC: fix template specialization in content::WebUI

GCC complains that explicit specialization in non-namespace scope
is happening for GetValue. Move the methods outside
the class definition.

Bug: 819294
Change-Id: I109472a0b6fa7ddab3529bc92bba680252b40f67
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2128307Reviewed-by: Camille Lamy <clamy@chromium.org>
Commit-Queue: Camille Lamy <clamy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#771818}

content/public/browser/web_ui.h

@@ -138,22 +138,6 @@ class CONTENT_EXPORT WebUI {
   template <typename T>
   static T GetValue(const base::Value& value);
 
-  template <>
-  inline bool GetValue<bool>(const base::Value& value) {
-    return value.GetBool();
-  }
-
-  template <>
-  inline int GetValue<int>(const base::Value& value) {
-    return value.GetInt();
-  }
-
-  template <>
-  inline const std::string& GetValue<const std::string&>(
-      const base::Value& value) {
-    return value.GetString();
-  }
-
   template <typename Is, typename... Args>
   struct Call;
 
@@ -169,6 +153,22 @@ class CONTENT_EXPORT WebUI {
   };
 };
 
+template <>
+inline bool WebUI::GetValue<bool>(const base::Value& value) {
+  return value.GetBool();
+}
+
+template <>
+inline int WebUI::GetValue<int>(const base::Value& value) {
+  return value.GetInt();
+}
+
+template <>
+inline const std::string& WebUI::GetValue<const std::string&>(
+    const base::Value& value) {
+  return value.GetString();
+}
+
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_BROWSER_WEB_UI_H_