DCHECK proper usage in CSSMathExpressionNode::DoubleValue()

CSSMathExpressionNode::DoubleValue() returns meaningless result if the
expression cannot be resolved into a single numeric value without any
type conversion. This patch adds DCHECK to ensure proper usage of this
function at runtime.

Bug: 979895
Change-Id: I8ddf0116bfd81d8248bec3cac5ecc08bb9a3903b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1693765
Commit-Queue: Emil A Eklund <eae@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#677234}
6 files changed