Add "number of cached WebKit UpdateAtlases" setting

:Release Notes:
Add setting for the WebKit2 Coordinated Graphics System UpdateAtlases.

:Detailed Notes:
- Added new setting numberOfUpdateAtlases - number of cached UpdateAtlases
which is used to render the content on them.

:Testing Performed:
That parameter takes effect when was combined with commits to WebKit and WAM2.

:QA Notes:
This change is required for:
- https://g2g.palm.com/#/c/2883/ (webappmanager2)
and relates to:
- https://gpro.palm.com/#/c/6489/ (webkit-starfish)

:Issues Addressed:
[GF-9858] [WAM2] Cache Update Atlases to speed up animations
[GF-7084] [Enyo Perf] Keep 3-4 UpdateAtlases cached

Open-webOS-DCO-1.0-Signed-off-by: Andrey Volykhin <andrey.volykhin@lge.com>

Change-Id: Ic7bb5445dbd03d8b433487c186eac22defeaf009
Reviewed-on: https://g2g.palm.com/2881
Tested-by: Andrey Volykhin <andrey.volykhin@lge.com>
Reviewed-by: Build Verification
Reviewed-by: Jose Dapena <jose.dapena@lge.com>
Reviewed-by: Andrey Volykhin <andrey.volykhin@lge.com>
Tested-by: Eric Blade <eric.blade@lge.com>
Reviewed-by: Eric Blade <eric.blade@lge.com>