bindings: make toImplArguments() less strict
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: bf5cd6ccde378db821296732a091cfbcf5285fbd@185366 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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