Small cleanup on BMPImageReader.cpp

'm_headerOffset + m_infoHeader.biSize' is used often in BMPImageReader.cpp.
It is even recalulated several time in a same function. This patch cleans up
this kind of repeated code.

BUG=

Review URL: https://codereview.chromium.org/872683008

git-svn-id: svn://svn.chromium.org/blink/trunk@189471 bbb929c8-8fbe-4397-9dbb-9b2b20218538
1 file changed