Allow default font size changing on Linux.

Allow default font size changing on Linux.

Merge note: This is a combined cherry-picks of (1) revert [1] +
(2) reland without problematic part [2], which results in reverting
only the problematic part of the original CL[3].

[1] https://codereview.chromium.org/1319613004
[2] https://codereview.chromium.org/1303163008
[3] https://codereview.chromium.org/549303004

NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
TBR=joleksy@opera.com, jochen@chromium.org, erg@chromium.org, jln@chromium.org
BUG=525176
----
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

Review URL: https://codereview.chromium.org/1303163008

Cr-Commit-Position: refs/heads/master@{#346353}
(cherry picked from commit b48e1b8c1aaf9b9ea0f397b57e979ba4384a09e2)

Revert of Allow default font size changing on Linux. (patchset #10 id:180001 of https://codereview.chromium.org/549303004/ )

Reason for revert:
This caused a regression:
https://code.google.com/p/chromium/issues/detail?id=525176

Original issue's description:
> 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/
>
> BUG=
>
> Committed: https://crrev.com/f0bc1a926d8f0326ef38b8374e42bc8e244dd2ba
> Cr-Commit-Position: refs/heads/master@{#344746}

>TBR=jochen@chromium.org,erg@chromium.org,eseidel@chromium.org,jln@chromium.org,dmazzoni@chromium.org,plundblad@chromium.org,joleksy@opera.com
>NOPRESUBMIT=true
>NOTREECHECKS=true
>NOTRY=true
>BUG=

Review URL: https://codereview.chromium.org/1319613004

Cr-Commit-Position: refs/heads/master@{#345852}
(cherry picked from commit 3b1be63348b61357f9dd1905f8a012fdf10bd523)

[kochi, 2015-09-01 04:52:30]

kochi@chromium.org changed reviewers:
- dmazzoni@chromium.org, eseidel@chromium.org, plundblad@chromium.org

[kochi, 2015-09-01 04:54:48]

The CQ bit was checked by kochi@chromium.org

[commit-bot: I haz the power, 2015-09-01 04:55:05]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1319463004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1319463004/1

[commit-bot: I haz the power, 2015-09-01 04:56:02]

Committed patchset #1 (id:1)