ASSERTION FAILED: !style->propertyIsImportant(propertyID) in WebCore::setTextDecorationProperty

When remove format command is called we pushdown the ancestor style
down to its children. Currently applying inline style to iframe
while pushing down style which causes iframe to be reinserted in tree and
triggres again subframe loading which repeats everytime and finally
crash happens. So we should avoid applying inline style to iframe
element as it does not reflect in its content while pushing down style
on it.

And ASSERT call has been removed from setTextDecoration property as
the scenario is perfectly valid case.

Backported from WebKit: http://trac.webkit.org/changeset/157710 (Patch by Santosh Mahto)

R=hayato@chromium.org
BUG=350943

Review URL: https://codereview.chromium.org/192373008

git-svn-id: svn://svn.chromium.org/blink/trunk@168900 bbb929c8-8fbe-4397-9dbb-9b2b20218538