md_browser: Switch to 4-space indent.

The Google Markdown style guide advocates 4-space indent[1].
When configured to use 2-space indent (what we were using
before), md_browser doesn't render 4-space-indented nested
lists or wrapped text correctly. Gitiles (used by Gerrit and
chromium.googlesource.com to render Markdown) also wants
4-space indent rather than 2-space indent.

1. https://github.com/google/styleguide/blob/gh-pages/docguide/style.md#nested-list-spacing

Bug: 736199
Change-Id: Icf4aa298f893672b6a7718aa1dc3e300f6559eff
Reviewed-on: https://chromium-review.googlesource.com/729749
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Dan Erat <derat@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#510349}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: db0868eb17e4f5a0ca928a14024f6ee8c061c0e8
diff --git a/md_browser.py b/md_browser.py
index 2052542..358a110 100755
--- a/md_browser.py
+++ b/md_browser.py
@@ -182,7 +182,7 @@
 
     md = markdown.Markdown(extensions=extensions,
                            extension_configs=extension_configs,
-                           tab_length=2,
+                           tab_length=4,
                            output_format='html4')
 
     has_a_single_h1 = (len([line for line in contents.splitlines()