[Telemetry] Fix up desktop_browser_finder on Linux

[Telemetry] Fix up desktop_browser_finder on Linux

This CL adds {stable, beta, dev} browsers on Linux, as well as fixes the bug
where if a user has stable installed but beta or dev default, browser
'system' would refer to stable unconditionally.

Unit tests have been expanded both to verify the bugfix and new features.

BUG=515286, 515295
R=aiolos@chromium.org,dtu,nednguyen
TEST=perf/run_benchmark --browser {system,stable,beta,dev} sunspider and
    telemetry/run_tests desktop_browser_finder

Committed: https://crrev.com/202b18768443abd15ddc6b5f47873f8e4415b38b
Cr-Commit-Position: refs/heads/master@{#341196}

[eakuefner, 2015-07-30 18:12:47]

PTAL

[nednguyen, 2015-07-30 20:06:45]

lgtm

https://codereview.chromium.org/1261863003/diff/1/tools/telemetry/telemetry/i...
File
tools/telemetry/telemetry/internal/backends/chrome/desktop_browser_finder.py
(right):

https://codereview.chromium.org/1261863003/diff/1/tools/telemetry/telemetry/i...
tools/telemetry/telemetry/internal/backends/chrome/desktop_browser_finder.py:246:
root = os.path.split(os.path.realpath('google-chrome'))[0]
Fix this

[eakuefner, 2015-07-30 20:16:31]

The CQ bit was checked by eakuefner@chromium.org

[eakuefner, 2015-07-30 20:16:31]

The patchset sent to the CQ was uploaded after l-g-t-m from nednguyen@google.com
Link to the patchset: https://codereview.chromium.org/1261863003/#ps40001
(title: "reflow")

[commit-bot: I haz the power, 2015-07-30 20:17:11]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1261863003/40001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1261863003/40001

[eakuefner, 2015-07-30 20:27:12]

https://codereview.chromium.org/1261863003/diff/1/tools/telemetry/telemetry/i...
File
tools/telemetry/telemetry/internal/backends/chrome/desktop_browser_finder.py
(right):

https://codereview.chromium.org/1261863003/diff/1/tools/telemetry/telemetry/i...
tools/telemetry/telemetry/internal/backends/chrome/desktop_browser_finder.py:246:
root = os.path.split(os.path.realpath('google-chrome'))[0]
On 2015/07/30 at 20:06:45, nednguyen wrote:
> Fix this

Done.

[commit-bot: I haz the power, 2015-07-30 21:29:19]

Committed patchset #3 (id:40001)

[commit-bot: I haz the power, 2015-07-30 21:30:00]

Patchset 3 (id:??) landed as
https://crrev.com/202b18768443abd15ddc6b5f47873f8e4415b38b
Cr-Commit-Position: refs/heads/master@{#341196}