Generate missing "block.ReThrow();" before return

After r174546 a v8::TryCatch object is in scope when calling a method's
C++ implementation early due to missing optional arguments.  If the
implementation throws an exception, we need to call ReThrow() on that
v8::TryCatch object for the exception to be propagated correctly.

BUG=362388

Review URL: https://codereview.chromium.org/307883002

git-svn-id: svn://svn.chromium.org/blink/trunk@175076 bbb929c8-8fbe-4397-9dbb-9b2b20218538