Use _window to get content height and width

[scott108]

It will get null reference content when run on release bundle mode, so I use _window to get ActualHight and ActualWidth, and it works fine

[msftclas]

@scott108,
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[msftclas]

@scott108,
Thanks for your contribution.
To ensure that the project team has proper rights to use your work, please complete the Contribution License Agreement at https://cla.microsoft.com.

It will cover your contributions to all Microsoft-managed open source projects.
Thanks,
Microsoft Pull Request Bot

[codecov-io]

Codecov Report

Merging #1264 into master will decrease coverage by 0.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1264      +/-   ##
==========================================
- Coverage   31.84%   31.83%   -0.02%     
==========================================
  Files         258      258              
  Lines       18674    18677       +3     
  Branches     1580     1583       +3     
==========================================
- Hits         5947     5945       -2     
- Misses      12574    12577       +3     
- Partials      153      155       +2

Impacted Files	Coverage Δ
...ative.Net46/Views/Scroll/ReactScrollViewManager.cs	0% <ø> (ø)	⬆️
...ative.Net46/Modules/DeviceInfo/DeviceInfoModule.cs	0% <0%> (ø)	⬆️
...dows/ReactNative.Shared.Tests/Internal/AssertEx.cs	84% <0%> (-4%)	⬇️
...ts/Modules/Network/TaskCancellationManagerTests.cs	93.7% <0%> (-0.7%)	⬇️

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