[MdBrowser] Set document encoding to UTF-8
[MdBrowser] Set document encoding to UTF-8

The md_browser.py script did not specify a content encoding. When
UTF-8 encoded markdown files are served via the script, Chrome assumed a
Windows 1252 encoding, displaying garble for non-ascii characters. This
is fixed by adding a charset meta tag to the template.

Note that Gitiles serves rendered markdown with UTF-8 encoding.

BUG=578855

Review URL: https://codereview.chromium.org/1601813004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#370130}

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