Add jessie support to install-build-deps.sh

This CL
- Enables jessie support in install-build-deps.sh
- Adds a note in linux_build_instructions.md about adding the
  'contrib' component in sources.list.
- Fixes a bug in package_exists where if a ppa is added, a package is
  installed from it, and then the ppa is removed, and the package is
  removed but not purged, package_exists would still return true for
  the package.

[email protected]

Review-Url: https://codereview.chromium.org/2556133003
Cr-Commit-Position: refs/heads/master@{#437198}
diff --git a/docs/linux_build_instructions.md b/docs/linux_build_instructions.md
index 957254e0..0fe97e6 100644
--- a/docs/linux_build_instructions.md
+++ b/docs/linux_build_instructions.md
@@ -334,14 +334,34 @@
 
 ### Debian
 
-`build/install-build-deps.sh` doesn't currently run on Debian, but you can
-probably hack it to get it to work. You will probably need to update the
-following package names:
+Some tests require the `ttf-mscorefonts-installer` package from the `contrib`
+component. `contrib` packages may have dependencies on non-free software.
 
-*   `libexpat-dev` → `libexpat1-dev`
-*   `freetype-dev` → `libfreetype6-dev`
-*   `libbzip2-dev` → `libbz2-dev`
-*   `libcupsys2-dev` → `libcups2-dev`
+If you need to run tests requiring MS TTF fonts, you can edit your apt
+`sources.list` by adding `contrib` to the end of each line beginning with `deb`.
+You might end up with something like this:
+
+```
+deb http://ftp.us.debian.org/debian/ jessie main contrib
+deb-src http://ftp.us.debian.org/debian/ jessie main contrib
+
+deb http://security.debian.org/ jessie/updates main contrib
+deb-src http://security.debian.org/ jessie/updates main contrib
+
+# jessie-updates, previously known as 'volatile'
+deb http://ftp.us.debian.org/debian/ jessie-updates main contrib
+deb-src http://ftp.us.debian.org/debian/ jessie-updates main contrib
+```
+
+Next, run:
+
+``` shell
+$ sudo apt-get update
+$ sudo apt-get install ttf-mscorefonts-installer
+```
+
+If you already have the `contrib` component enabled, `install-build-deps.sh`
+will install `ttf-mscorefonts-installer` for you.
 
 ### Fedora