Avoid repeated command-line checks for forced DPI scale

Avoid repeated command-line checks for forced DPI scale

Every time |Screen.GetPrimaryDisplay()| is accessed on Android, which can be
multiple times a frame, we're querying the command-line for
kForceDeviceScaleFactor.  Instead, cache the result of the query, saving ~8us
(50% of the cost) per GetPrimaryDisplay() call on a typical Android device.

BUG=341613
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=255524

[jdduke (slow), 2014-03-06 21:04:07]

oshima@: PTAL.  I was slightly surprised at how costly the command-line query
was (for comparison, a synchronized lock takes <1us on the same device).

[oshima, 2014-03-06 22:01:06]

lgtm

Are we checking this often? I assumed this is called only when some change is
made to display configuration.

[jdduke (slow), 2014-03-06 22:02:33]

On 2014/03/06 22:01:06, oshima wrote:
> lgtm
> 
> Are we checking this often? I assumed this is called only when some change is
> made to display configuration.

It appears to be commonly used on Android to get the device scaling factor. 
It's really a sad state of affairs, as we really ought to be listening to
changes in these values rather than just pushing them around in a somewhat
ad-hoc fashion.

[oshima, 2014-03-06 22:04:58]

On 2014/03/06 22:02:33, jdduke wrote:
> On 2014/03/06 22:01:06, oshima wrote:
> > lgtm
> > 
> > Are we checking this often? I assumed this is called only when some change
is
> > made to display configuration.
> 
> It appears to be commonly used on Android to get the device scaling factor. 
> It's really a sad state of affairs, as we really ought to be listening to
> changes in these values rather than just pushing them around in a somewhat
> ad-hoc fashion.

Ah i see. Thank you for identifying and fixing it anyway.

[jdduke (slow), 2014-03-06 22:26:33]

The CQ bit was checked by jdduke@chromium.org

[commit-bot: I haz the power, 2014-03-06 22:33:29]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/jdduke@chromium.org/186733009/20001

[commit-bot: I haz the power, 2014-03-07 04:23:47]

Change committed as 255524