grit: add video/webm mime type.

It turns out that video/webm mime type is not always available
(depending on the python version?) So this Cl adds it statically.

Bug: 852038
Change-Id: Ic8cb19a777131df96df6c5794598ea8c8d0d312a
Reviewed-on: https://chromium-review.googlesource.com/1168655Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Alexander Alekseev <alemate@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581838}

tools/grit/grit/format/html_inline.py

@@ -28,6 +28,9 @@ from grit.format import minifier
 mimetypes.init([])
 mimetypes.add_type('image/svg+xml', '.svg')
 
+# webm video type is not always available if mimetype package is outdated.
+mimetypes.add_type('video/webm', '.webm')
+
 DIST_DEFAULT = 'chromium'
 DIST_ENV_VAR = 'CHROMIUM_BUILD'
 DIST_SUBSTR = '%DISTRIBUTION%'