Improve computation of ctmScale{X,Y} in NativeImageSkia::drawPattern

Simply using "scale x" and "scale y" will not give the intended results
for for instance mirroring transformations.
Instead compute the length of the column vectors for the 2x2 submatrix.

BUG=160443

Review URL: https://codereview.chromium.org/223673003

git-svn-id: svn://svn.chromium.org/blink/trunk@170782 bbb929c8-8fbe-4397-9dbb-9b2b20218538