tools/md_browser: Add .svg preview support.

gitiles supports svg, according to
https://github.com/google/gitiles/blob/master/gitiles-servlet/src/main/resources/com/google/gitiles/mime-types.properties

Bug: 
Change-Id: I87e54da98d434b0a932349b682f35e53a6d640a8
Reviewed-on: https://chromium-review.googlesource.com/722425Reviewed-by: Nodir Turakulov <nodir@chromium.org>
Commit-Queue: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509432}

tools/md_browser/md_browser.py

@@ -153,6 +153,8 @@ class Handler(SimpleHTTPServer.SimpleHTTPRequestHandler):
       self._DoImage(full_path, 'image/png')
     elif path.lower().endswith('.jpg'):
       self._DoImage(full_path, 'image/jpeg')
+    elif path.lower().endswith('.svg'):
+      self._DoImage(full_path, 'image/svg+xml')
     elif os.path.isdir(full_path):
       self._DoDirListing(full_path)
     elif os.path.exists(full_path):