Revert of Remove unused FontPlatformDataHarfBuzz::emSizeInFontUnits

Revert of Remove unused FontPlatformDataHarfBuzz::emSizeInFontUnits (https://codereview.chromium.org/210573002/)

Reason for revert:
This might have caused android tests failure: 
http://build.chromium.org/p/chromium.webkit/builders/Android%20Tests%20%28dbg%29/builds/18266
Speculatively reverting it.

Original issue's description:
> Remove unused FontPlatformDataHarfBuzz::emSizeInFontUnits
> 
> Remove the unused m_emSizeInFontUnits field and emSizeInFontUnits
> method from FontPlatformDataHarfBuzz.
> 
> BUG=355806
> R=pdr@chromium.org
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=169955

TBR=pdr@chromium.org,dglazkov@chromium.org,eae@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=355806
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=170048

[vsevik, 2014-03-26 12:10:35]

Created Revert of Remove unused FontPlatformDataHarfBuzz::emSizeInFontUnits

[commit-bot: I haz the power, 2014-03-26 12:10:41]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/vsevik@chromium.org/212763002/1

[commit-bot: I haz the power, 2014-03-26 12:10:55]

Change committed as 170048

[vsevik, 2014-03-26 12:22:31]

A revert of this CL has been created in
https://codereview.chromium.org/212563005/ by vsevik@chromium.org.

The reason for reverting is: I don't think this was a reason of failures.