Split javascript_dialogs.cc into views and cocoa+views versions.

This allows them both to be compiled on Mac, which is needed for
mac_views_browser (since it doesn't link any Cocoa code).

Compile step regressed at go/macviewsbuilder/builds/16832.

BUG=629964, 425229

Review-Url: https://codereview.chromium.org/2458903002
Cr-Commit-Position: refs/heads/master@{#428316}
3 files changed