Script streaming: detect encoding later.

It's possible that Resource::encoding() changes once we receive data.

This requires moving the creation of v8::ScriptCompiler::StreamedSource and
SourceStream forward in time (we need to know the encoding to create them).

Additional fix: m_cachedDataType was not initialized properly.

BUG=428137

Review URL: https://codereview.chromium.org/700543002

git-svn-id: svn://svn.chromium.org/blink/trunk@184826 bbb929c8-8fbe-4397-9dbb-9b2b20218538