RFR: JDK-8258659: Eliminate whitespace comments from generated pages [v2]

[Jonathan Gibbons]

> Empty comments were being used to stop empty nodes being optimized away in `HtmlTree.add`, but the empty comments caused annoying noise in the generated docs.  A better solution is to use `HtmlTree.EMPTY`, which had the right effect, without the unwanted side-effects.

Jonathan Gibbons has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains two additional commits since the last revision:

 - Merge remote-tracking branch 'upstream/master' into empty-comment
 - JDK-8258659: Eliminate whitespace comments from generated pages

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1836/files
  - new: https://git.openjdk.java.net/jdk/pull/1836/files/c86279d4..a09bb49d

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1836&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1836&range=00-01

  Stats: 39223 lines in 1392 files changed: 14862 ins; 11904 del; 12457 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1836.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1836/head:pull/1836

PR: https://git.openjdk.java.net/jdk/pull/1836