Make screen dimension reflection fast
Make screen dimension reflection fast

Summary:
After loading a layout plugin snapshot, I noticed the plugin feels really sluggish. I noticed that getDimensions, which recursively walks the full tree, is triggered on every rendering, although the state it introduces is only required when opening a screenshot visualizer. So Moved the function invocation to there.

Also did some first steps on supporting iOS; the visualizer will no show a preview of iOS as well. I'll leave implementing the highlights to a volunteer :)

Reviewed By: passy

Differential Revision: D29262697

fbshipit-source-id: b13b19b013eeeb98abb66fb648a193c69688a7e3

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