Reorder and group method declarations and add one comment for readability

responseType related methods are reordered so that accessor goes first
to be consistent with the other methods.

Added a comment about internalAbort() explaining that the method may
trigger V8 GC.

BUG=none

Review URL: https://codereview.chromium.org/667203003

git-svn-id: svn://svn.chromium.org/blink/trunk@184143 bbb929c8-8fbe-4397-9dbb-9b2b20218538