docs: Update Fedora dependencies in Linux build instructions.

This CL points to an updated method for installing the fonts needed by
LayoutTests on Fedora. It also adds ixorg-x11-server-Xvfb to the list of
required packages, as Blink's run-webkit-tests tool requires xvfb.

Bug: 
Change-Id: I8bf9a1539066880ca0ea450786847a5126f9cefb
Reviewed-on: https://chromium-review.googlesource.com/764861Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#516016}

docs/linux_build_instructions.md

@@ -385,15 +385,15 @@ libcap-devel libffi-devel libgcc.i686 libgnome-keyring-devel libjpeg-devel \
 libstdc++.i686 libX11-devel libXScrnSaver-devel libXtst-devel \
 libxkbcommon-x11-devel ncurses-compat-libs nspr-devel nss-devel pam-devel \
 pango-devel pciutils-devel pulseaudio-libs-devel zlib.i686 httpd mod_ssl \
-php php-cli python-psutil wdiff'
+php php-cli python-psutil wdiff xorg-x11-server-Xvfb'
 ```
 
-The `msttcorefonts` packages can be obtained by following [these
-instructions](http://www.fedorafaq.org/#installfonts). For the optional
-packages:
+The fonts needed by Blink's LayoutTests can be obtained by following [these
+instructions](https://gist.github.com/pwnall/32a3b11c2b10f6ae5c6a6de66c1e12ae).
+For the optional packages:
 
 * `php-cgi` is provided by the `php-cli` package.
-* `sun-java6-fonts` doesn't exist in Fedora repositories, needs investigating.
+* `sun-java6-fonts` is covered by the instructions linked above.
 
 ### Gentoo