Regression(r153187): Crash reading webp image color profile

Based on a patch by Pascal Massimino.

setData() may have shuffled the encoded data pointers inbetween the
creation of m_demux object and its use.  Ensure m_demux is properly
updated prior to reading color profile data.

TBR=inferno@chromium.org
BUG=255931

Review URL: https://chromiumcodereview.appspot.com/18691002

git-svn-id: svn://svn.chromium.org/blink/trunk@153598 bbb929c8-8fbe-4397-9dbb-9b2b20218538