fix(textfield): add placeholder mixins and disabled placeholder color

[asyncliz]

No description provided.

[allan-chen]

Nice, left some minor comments

[mdc-web-bot]

🤖 Beep boop!

Screenshot test report 🚦

12 screenshots changed from master on commit 3e32120:

• https://storage.googleapis.com/mdc-web-screenshot-tests/travis/2019/12/18/19_59_20_138/report/report.html?utm_source=github&utm_medium=pr_comment

Details

6 Changed:

• spec/mdc-textfield/classes/baseline-placeholder.html
• spec/mdc-textfield/mixins/disabled-1.html

6 Added:

• spec/mdc-textfield/classes/disabled-placeholder.html

• spec/mdc-textfield/mixins/disabled-3.html

[allan-chen]

LGTM, let's sync to internal

[codecov-io]

Codecov Report

Merging #5355 into master will decrease coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5355      +/-   ##
==========================================
- Coverage   98.39%   98.31%   -0.08%     
==========================================
  Files         163      163              
  Lines        6307     6309       +2     
  Branches      863      864       +1     
==========================================
- Hits         6206     6203       -3     
- Misses        101      106       +5

Impacted Files	Coverage Δ
packages/mdc-radio/component.ts	87.5% <0%> (-8.16%)	⬇️
packages/mdc-radio/foundation.ts	81.81% <0%> (-4.55%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update bac43eb...3a4195c. Read the comment docs.

[googlebot]

All (the pull request submitter and all commit authors) CLAs are signed, but one or more commits were authored or co-authored by someone other than the pull request submitter.

We need to confirm that all authors are ok with their commits being contributed to this project. Please have them confirm that by leaving a comment that contains only @googlebot I consent. in this pull request.

Note to project maintainer: There may be cases where the author cannot leave a comment, or the comment is not properly detected as consent. In those cases, you can manually confirm consent of the commit author(s), and set the cla label to yes (if enabled on your project).

ℹ️ Googlers: Go here for more info.

[googlebot]

CLAs look good, thanks!

ℹ️ Googlers: Go here for more info.

[mdc-web-bot]

🤖 Beep boop!

Screenshot test report 🚦

12 screenshots changed from master on commit 3a4195c:

• https://storage.googleapis.com/mdc-web-screenshot-tests/travis/2019/12/18/23_36_31_374/report/report.html?utm_source=github&utm_medium=pr_comment

Details

9 Changed:

• spec/mdc-textfield/classes/baseline-placeholder.html
• spec/mdc-textfield/mixins/disabled-1.html
• spec/mdc-textfield/mixins/disabled-2.html

3 Added:

• spec/mdc-textfield/classes/disabled-placeholder.html