IDL: Merge duplicating local variable assignment in attribute_getter

|cpp_type| can be non-*Ptr type so it seems that we can't always use
local variables if we want to merge local variable assignment.
Following Jens' suggestion[1] to merge local variable assignment.

[1] https://codereview.chromium.org/359703002/diff/1/Source/bindings/templates/attributes.cpp#newcode51

BUG=345503

Review URL: https://codereview.chromium.org/351403003

git-svn-id: svn://svn.chromium.org/blink/trunk@177090 bbb929c8-8fbe-4397-9dbb-9b2b20218538