Allow default font size changing on Linux.

Allow default font size changing on Linux.

Currently base rendering font size on Linux is hardcoded to 16.0 with no possibility of changing it. This patch add such possibility in RendererPreferences.

Webkit part: https://codereview.chromium.org/570243002/

This was once landed and reverted:
https://codereview.chromium.org/549303004 (original)
https://codereview.chromium.org/1319613004 (revert)

due to unexpected default font size change.

Now this CL removes the problematic part to derive the default font
size from desktop environment.

BUG=525176
TBR=joleksy@opera.com, jochen@chromium.org, erg@chromium.org, jln@chromium.org

Committed: https://crrev.com/b48e1b8c1aaf9b9ea0f397b57e979ba4384a09e2
Cr-Commit-Position: refs/heads/master@{#346353}

[kochi, 2015-08-31 06:02:55]

The CQ bit was checked by kochi@chromium.org

[commit-bot: I haz the power, 2015-08-31 06:03:01]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1303163008/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1303163008/1

[kochi, 2015-08-31 06:10:30]

kochi@chromium.org changed reviewers:
+ joleksy@opera.com
- jolesky@opera.com

[kochi, 2015-08-31 06:10:56]

Now with correct TBR addresses.

[commit-bot: I haz the power, 2015-08-31 07:32:37]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-08-31 07:33:09]

Patchset 1 (id:??) landed as
https://crrev.com/b48e1b8c1aaf9b9ea0f397b57e979ba4384a09e2
Cr-Commit-Position: refs/heads/master@{#346353}