X Selection: Refactoring (range-based for loops, NULL to nullptr)

- Only in ui/base/x/*
- Updated X Selection code to use range-based for loops.
- Updated uses of NULL to nullptr
- `git cl format` also updated some formatting.

Bug: 896479
Change-Id: I5fea75df87872341d726e2f7bbeb0d93c0a89b61
Reviewed-on: https://chromium-review.googlesource.com/c/1368944
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Commit-Queue: Darwin Huang <huangdarwin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615287}
5 files changed