Use reference instead of pointer in BMPImageDecoder::processFileHeader

Use reference instead of pointer in BMPImageDecoder::processFileHeader

Review URL: https://codereview.chromium.org/556913002

git-svn-id: svn://svn.chromium.org/blink/trunk@181777 bbb929c8-8fbe-4397-9dbb-9b2b20218538