Use more appropriate symbol names for "shadow distributed"

The term "shadow distributed" has been used to refer
to matching of "::content" pseudo element against
<content> or <shadow> elements.

Shadow DOM V1 introduces the new <slot> element and
::slotted pseudo element, rename all occurrences
of "shadow distributed" to "content pseudo", or
"slotted pseudo" etc. so it should make more sense.

This CL should land after
https://codereview.chromium.org/1575363006

BUG=531990

Review URL: https://codereview.chromium.org/1616743002

Cr-Commit-Position: refs/heads/master@{#370660}
5 files changed