Track if textLength spacing is in effect while walking the line box tree

Track if textLength spacing is in effect while walking the line box tree

Instead of calling parentDefinesTextLength() in
layoutTextOnLineOrPath(), keep a flag updated as we traverse the line
box tree. This eliminates the walk up the ancestor-chain, but more
importantly it avoids checking this for each text box.
To make this easier, layoutCharactersInTextBoxes() is moved from
SVGRootInlineBox to SVGTextLayoutEngine. This allows simplifying
SVGTextLayoutEngine::beginTextPathLayout a bit to make it more
independent.
Gives roughly a 1% speedup on separate-x.svg from the bug, when run
through PerfTestRunner.measurePageLoadTime.

BUG=486669
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=196158

[fs, 2015-05-29 15:49:35]

fs@opera.com changed reviewers:
+ fmalita@chromium.org, pdr@chromium.org

[fs, 2015-05-29 15:49:36]

A few more cycles bites the dust.

[pdr., 2015-05-29 16:12:49]

LGTM

[f(malita), 2015-05-29 16:26:05]

lgtm

[f(malita), 2015-05-29 16:27:08]

The CQ bit was checked by fmalita@chromium.org

[commit-bot: I haz the power, 2015-05-29 16:27:23]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1158033006/1

[commit-bot: I haz the power, 2015-05-29 16:30:32]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=196158