bindings: make toImplArguments() less strict

Allow it to be called with startIndex that is greater than info.Length().
This will happen from the generated code for a method such as

  void f(long x, optional long y = 0, long... rest);

when called with a single argument.

Also a minor improvement: reserve a tighter initial capacity for the
result vector. Capacity used to be reserved for storing all arguments,
disregarding the startIndex value.

Review URL: https://codereview.chromium.org/732483002

git-svn-id: svn://svn.chromium.org/blink/trunk@185366 bbb929c8-8fbe-4397-9dbb-9b2b20218538