Reland "Change fallback font collection in HarfBuzzShaper"

Reland "Change fallback font collection in HarfBuzzShaper"

Change HarfBuzzShaper to not populate the fallback font list during text
shaping and instead populate it based on the ShapeResult objects at call
time. This circumvents additional registration work and renders the text
shaping API simpler and more consistent.

Originally landed in r198813 (bbb929c88fbe43979dbb9b2b20218538) and then
reverted in r198818 (64e30826c261ac7a2607e891dbe7e9d2d251e5f3) by virtue
of Mac 10.6 test failures caused by a missing NULL check, since added.

This is a speculative fix.

R=leviw@chromium.org

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=198886

[eae, 2015-07-14 18:54:00]

eae@chromium.org changed reviewers:
+ leviw@chromium.org

[leviw_travelin_and_unemployed, 2015-07-14 19:36:51]

Can you change the description to mention that this is a speculative fix?

LGTM.

[eae, 2015-07-14 19:59:41]

The CQ bit was checked by eae@chromium.org

[commit-bot: I haz the power, 2015-07-14 20:00:10]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1241613006/1

[commit-bot: I haz the power, 2015-07-14 20:04:03]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=198886

[Noel Gordon, 2015-07-15 04:36:34]

A revert of this CL (patchset #1 id:1) has been created in
https://codereview.chromium.org/1235163007/ by noel@chromium.org.

The reason for reverting is: Broke Linux MSAN bots.

http://build.chromium.org/p/chromium.webkit/builders/WebKit%20Linux%20MSAN/bu...

See stdio log of 4412 build: many reports use on an uninitialized variable in
shaping font fallback, about 100+ tests failing that way..