Currently on Mac and iOS, gfx::NativeViewAccessible is 'typedef'ed to void*
or id depending on whether the file it's used in is an Objective C file or not
(i.e. if __OBJC__ is defined or not). Previously, when it was only ever used as
a return type on these platforms, this was fine. But in order to implement
AXPlatformNode::FromNativeViewAccessible() on Mac where
gfx::NativeViewAccessible is used as an method argument, this no longer works --
.cc and .mm files see different method signatures, and linking these together
fails (since the .mm file will declare an overload that doesn't exist).

To fix, typedef gfx::NativeViewAccessible to id.

BUG=610589

Review-Url: https://codereview.chromium.org/2739783002
Cr-Commit-Position: refs/heads/master@{#456594}