Refactor: unsigned short/unsigned -> uint16_t in third_party/blink/renderer/platform/text

- unsigned short / unsigned -> uint16_t.
- USHRT_MAX -> std::numeric_limits<uint16_t>::max()
- Reference: https://google.github.io/styleguide/cppguide.html#Integer_Types

Bug: 929986
Change-Id: If80935d74042f4f8e50275d0ec17832e19ae7046
Reviewed-on: https://chromium-review.googlesource.com/c/1490367
Commit-Queue: Miyoung Shin <myid.shin@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636641}
3 files changed