[XHR] Explain that m_loader is sufficient to represent "send() flag" in abort()

The variable corresponds to the spec term "send() flag" in Blink code is
sendFlag. It's obtained by checking m_loader is null or not. This doesn't
work when m_async is false. But it's ok since we are never able to
call abort() when the XHR in the state where abort() algorithm needs to
look into "send() flag".

This CL adds explanation of that, add m_async check in the if-clause,
and rewrite current sophisticated condition for checking state to
simpler one. The new condition is equivalent to the old one and is also
closer to the condition in natural language in the spec text.

BUG=none

Review URL: https://codereview.chromium.org/559373002

git-svn-id: svn://svn.chromium.org/blink/trunk@182218 bbb929c8-8fbe-4397-9dbb-9b2b20218538