WebUI cleanup: Rename IDR_MD_EXTENSIONS_ to IDR_EXTENSIONS_.

This is part of the effort to remove unnecessary "md_" prefixes from various
parts of the WebUI codebase.

Bug: 887150
Change-Id: I5769e7064e8acd359a3293a42a3bded90072d0c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1559460
Auto-Submit: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Esmael El-Moslimany <aee@chromium.org>
Reviewed-by: Esmael El-Moslimany <aee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#649248}
3 files changed