Carets in GMail and iCloud compositions are the foreground text color

Carets in GMail and iCloud compositions are the foreground text color
https://bugs.webkit.org/show_bug.cgi?id=138029

Reviewed by Simon Fraser.

Source/WebCore:

Previously, we were only checking whether the background colors of the foreground and background
elements were the same, rather than taking validity and opacity into consideration.

Test: editing/caret/color-span-inside-editable-background.html

* editing/FrameSelection.cpp:
(WebCore::disappearsIntoBackground):
(WebCore::CaretBase::paintCaret):

LayoutTests:

Test the case where the content editable root has a background color specified.

* editing/caret/color-span-inside-editable-background-expected.html: Added.
* editing/caret/color-span-inside-editable-background.html: Added.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@175152 268f45cc-cd09-0410-ab3c-d52691b4dbfc