Bug 189298

Update code style guidelines for using namespace inside a namespace definition

https://webkit.org/code-style-guidelines/#using-position

> [](#using-position) In implementation files, put all other "using" statements at the beginning of the file, before any namespace definitions and after any "include" statements.
> 
> ###### Right:
> 
> ```cpp
> // HTMLSelectElement.cpp
> 
> using namespace other;
> 
> namespace WebCore {
> 
> } // namespace WebCore
> ```
> 
> ###### Wrong:
> 
> ```cpp
> // HTMLSelectElement.cpp
> 
> namespace WebCore {
> 
> using namespace other;
> 
> } // namespace WebCore
> ```


[webkit-dev] Moving WebCore to unified source builds and style changes.
https://lists.webkit.org/pipermail/webkit-dev/2017-October/029656.html

> 1) Move all “using namespace <name>;” inside the WebCore namespace (they used to be in the global namespace) and change to "using WebCore::<type-name>;” in .mm files.