[jdk11u-dev] RFR: 8220202: Simplify/standardize method naming for HtmlTree

[Amos Shi]

Backport of [JDK-8220202](https://bugs.openjdk.org/browse/JDK-8220202)
- This PR contains 2 commits
- Commit 1: is the git apply from the original commit
- Commit 2: is trying to aplly the rejected changes. Although it contains many files, well it only contains 3 types of changes:
  - Change type A. Change method name from `addContent` to `add`
  - Change type B. Change method name from `addAttr` to `put`
  - Change type C. Add missing `import` to fix compile error

Testing
- Local: Test passed
  - `TestHtmlDocument.java`: Test results: passed: 1
- Pipeline: 
- Testing Machine:

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Commit messages:
 - Backport 1b9db9b7a131f7cf24b0ca64337ff61ad3544ef5
 - Backport 1b9db9b7a131f7cf24b0ca64337ff61ad3544ef5

Changes: https://git.openjdk.org/jdk11u-dev/pull/2674/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=2674&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8220202
  Stats: 1225 lines in 83 files changed: 2 ins; 6 del; 1217 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/2674.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev.git pull/2674/head:pull/2674

PR: https://git.openjdk.org/jdk11u-dev/pull/2674