Apply flood-opacity for feDropShadow

FEDropShadow::m_shadowOpacity was set but never used.

Also fix a typo nearby in FEDropShadow::createImageFilter (stdY should
be scaled with the vertical scale factor - as in
FEDropShadow::applySoftware.)

TEST=svg/filters/feDropShadow-flood-opacity-*.svg
BUG=336222

Review URL: https://codereview.chromium.org/143463002

git-svn-id: svn://svn.chromium.org/blink/trunk@165445 bbb929c8-8fbe-4397-9dbb-9b2b20218538