Xcode: Add .xcassets = folder.assetcatalog in extension_map
Xcode: Add .xcassets = folder.assetcatalog in extension_map

This allows lastKnownFileType to be set for files named, for instance,
Images.xcassets.

Patch by Matthew DeVore <matvore@google.com>
Review URL: https://codereview.chromium.org/140103003/

witwall/gyp,witwall/gyp,witwall/gyp,witwall/gyp,witwall/gyp