Fix more disabled MSVC warnings, Source/wtf/dtoa* edition.

Mostly, this is about fixing "possible value truncation", usually when assigning
a larger type to a smaller one.

It was not always clear to me that certain (currently implied) casts were safe; please review those with particular care and let me know whether to remove the comment, add an assertion, or add handler code as relevant.

BUG=81439
TEST=none

Review URL: https://codereview.chromium.org/519613002

git-svn-id: svn://svn.chromium.org/blink/trunk@183092 bbb929c8-8fbe-4397-9dbb-9b2b20218538