Update the build instructions to be consistent.

docs/linux_arch_build_instructions.md

+# Building Chromium on Arch Linux
+
+Generally speaking, follow the [Linux Build Instructions](linux_build_instructions.md).
+However, do the following instead to install the build dependencies:
+
+## Install the build dependencies:
+
+Most of these packages are probably already installed since they're often used,
+and the parameter --needed ensures that packages up to date are not reinstalled.
+
+    sudo pacman -S --needed python perl gcc gcc-libs bison flex gperf pkgconfig \
+    nss alsa-lib gconf glib2 gtk2 nspr ttf-ms-fonts freetype2 cairo dbus \
+    libgnome-keyring
+
+For the optional packages on Arch Linux:
+
+*   php-cgi is provided with pacman
+*   wdiff is not in the main repository but dwdiff is. You can get wdiff in
+    AUR/yaourt
+*   sun-java6-fonts do not seem to be in main repository or AUR.