IDL: Use ALLOW_ONLY_INLINE_ALLOCATION() in dictionaries

Before this CL, we allocate memory for IDL dictionaries each time we use
them. For performance sensitive methods, this may not be acceptable.
Avoid allocating them in the oilpan heap. Instead, put them on stack.
Dictionaries can be a member of collections, so we use
ALLOW_ONLY_INLINE_ALLOCATION().

Side effect of this change:
If an impl class implements an IDL method which returns a dictionary,
the impl method needs to take an argument for return value.

This CL also add blank lines in generated dictionary impl classes for
readability.

BUG=321462

Review URL: https://codereview.chromium.org/656073002

git-svn-id: svn://svn.chromium.org/blink/trunk@183855 bbb929c8-8fbe-4397-9dbb-9b2b20218538