Implement Element.localName.(fixes #2188)

[lpy]

see #2188

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/1350

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[highfive]

Warning

• These commits modify layout code, but no reftests are modified. Please consider adding a reftest!
• @lpy, please confirm that src/test/html/acid1.html and your favourite wikipedia page still render correctly!

[bors-servo]

saw approval from Ms2ger
at https://github.com/lpy/servo/commit/dfe5215b8876fc0f3b4ed01e989ef9b0e129c094

[bors-servo]

merging lpy/servo/issue2188 = dfe5215 into auto

[bors-servo]

lpy/servo/issue2188 = dfe5215 merged ok, testing candidate = 3fc2c11

[bors-servo]

all tests pass:
success: http://servo-buildbot.pub.build.mozilla.org/builders/linux/builds/1265
success: http://servo-buildbot.pub.build.mozilla.org/builders/mac/builds/1219

[bors-servo]

fast-forwarding master to auto = 3fc2c11