[XHR] Move bools to end of class declaration for better padding

Move all 7 bool members of class XHRHttpRequest to end of declaration.
This results in better padding and reduces class size by 16 bytes.

Review URL: https://codereview.chromium.org/346033003

git-svn-id: svn://svn.chromium.org/blink/trunk@176859 bbb929c8-8fbe-4397-9dbb-9b2b20218538