Add a flag for fast text autosizing

Add a flag for fast text autosizing

This is the third of three patches to add a runtime flag for fast text
autosizing. This patch adds the Content-side plumbing to enable
the Blink-side feature.

This patch depends on https://codereview.chromium.org/25452002 landing.

BUG=302005
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=226525

[eseidel, 2013-10-01 18:24:11]

LGTM, but I don't think I have sufficient powers.

[eseidel, 2013-10-01 18:26:22]

I normally ask avi, piman or jam for my content rubber-stamps.

[jam, 2013-10-01 18:39:55]

lgtm

[commit-bot: I haz the power, 2013-10-01 18:41:11]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/pdr@chromium.org/25454002/1

[commit-bot: I haz the power, 2013-10-01 22:54:41]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/pdr@chromium.org/25454002/1

[commit-bot: I haz the power, 2013-10-01 23:06:47]

Sorry for I got bad news for ya.
Compile failed with a clobber build on android_dbg.
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=android_db...
Your code is likely broken or HEAD is junk. Please ensure your
code is not broken then alert the build sheriffs.
Look at the try server FAQ for more details.

[commit-bot: I haz the power, 2013-10-02 03:00:42]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/pdr@chromium.org/25454002/1

[commit-bot: I haz the power, 2013-10-02 03:16:29]

Sorry for I got bad news for ya.
Compile failed with a clobber build on linux_clang.
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=linux_clan...
Your code is likely broken or HEAD is junk. Please ensure your
code is not broken then alert the build sheriffs.
Look at the try server FAQ for more details.

[commit-bot: I haz the power, 2013-10-02 16:38:19]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/pdr@chromium.org/25454002/1

[commit-bot: I haz the power, 2013-10-02 19:25:01]

Change committed as 226525