Make the Vector copy constructor for mismatched inline sizes explicit.

This exposes a bunch of existing places where an implicit conversion was
occurring. These have been updated to explicit conversions.

Review URL: https://chromiumcodereview.appspot.com/23903041

git-svn-id: svn://svn.chromium.org/blink/trunk@157657 bbb929c8-8fbe-4397-9dbb-9b2b20218538