Add settings for tuning WebKit tiled backing store

:Release Notes:
Added settings for the webkit TiledBackingStore tile size, and for
disabling power of two dimension enforcement in UpdateAtlas allocation.

:Detailed Notes:
- Added new settings tileWidth and TileHeight in General section, for
  changing the default tile sizes in TiledBackingStore.
- Added new setting nonPowerOfTwoUpdateAtlas, for disabling power of
  two enforcement on the area allocation of UpdateAtlas.

:Testing Performed:
Tried combined with webappmanager2 change and webkit2 change, and the
parameters take effect. For checking it I used the environment variable
WEBKIT_SHOW_COMPOSITING_DEBUG_VISUALS=1

:QA Notes:

:Issues Addressed:
[GF-8469] [Memory] Use an optimal backing store tile size on wam2

Open-webOS-DCO-1.0-Signed-off-by: Jose Dapena Paz <jose.dapena@lge.com>

Change-Id: I66de4f1c75100ed9c5236fc80a3cc644c9238854
Reviewed-on: https://g2g.palm.com/2835
Reviewed-by: Build Verification
Reviewed-by: Jose Dapena <jose.dapena@lge.com>
Tested-by: Jose Dapena <jose.dapena@lge.com>
Reviewed-by: Alexander Irhin <alexander.irhin@lge.com>
Reviewed-by: Eric Blade <eric.blade@lge.com>