Get the linen background painting during elastic scrolling

We weren't setting the location of the resource bundle correctly in the
renderer process. It turns out base::mac::OuterBundle() returns the helper
app's bundle in the renderer process. So now we have MainApplicationBundle() to
give us the bundle of the main app.
Enforce better header hygiene

Apple recommends disabling ALWAYS_SEARCH_USER_PATHS, so we now do. And we also
disable USE_HEADER_MAP to match other platforms more closely.
Fix a NSSplitView leak