Begin making LayoutUnit conversions to other types explicit

LayoutUnit misuse, often from improper conversion to other types, is
a constant source of bugs in Blink. This is the first patch with the
goal of making such error-prone conversions explicit in the hopes of
forcing developers to think through these conversions while writing
their patches. These conversion sites were discovered by toggling the
float and double conversions to explicit and finding the compiler errors.

BUG=350474

Review URL: https://codereview.chromium.org/191053004

git-svn-id: svn://svn.chromium.org/blink/trunk@168798 bbb929c8-8fbe-4397-9dbb-9b2b20218538