[fuchsia][scenic] Fix logical size in flatland platform view.

[shivesh-ganju]

This PR fixes a bug identified after DPR feature was added in flatland where currently we do not multiple the logical size of the view by the pixel ratio.

This change is getting tested in input tests like mouse-input-test, touch-input-test

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See testing the engine for instructions on
  writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

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[chinmaygarde]

@akbiggs Did you mean to remove the autosubmit tag? Looks like this PR is good to go.