DevTools: Do not count inspectElement time if drawer was shown

BUG=

Review URL: https://codereview.chromium.org/182973003

git-svn-id: svn://svn.chromium.org/blink/trunk@169146 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Fix bug in XMLDocumentParser.cpp.

There is no need to add |m_currentOffset| to |data| since |data| already points
to the begining of the buffer + |m_currentOffset|.

This CL also moves |OffsetBuffer| to its own file and adds a set of unit tests.

BUG=351720

Review URL: https://codereview.chromium.org/196633006

git-svn-id: svn://svn.chromium.org/blink/trunk@169147 bbb929c8-8fbe-4397-9dbb-9b2b20218538