Fix comment in fx_string.h

Fix comment in fx_string.h

Previous CLs have removed or marked explicit the constructors
which allowed bad constructs to compile.  I now get an error:
candidate constructor not viable: expects an l-value for 1st argument
when called as indicated in the comment.

Committed: https://pdfium.googlesource.com/pdfium/+/ade9465067098d9f94a13f61741cebf4bb8aac47

[Tom Sepez, 2016-05-12 21:44:02]

tsepez@chromium.org changed reviewers:
+ thestig@chromium.org

[Tom Sepez, 2016-05-12 21:44:03]

Lei, only the comment changed.  Payoff from some of the "explict" work.

[Lei Zhang, 2016-05-12 22:09:41]

lgtm

[Tom Sepez, 2016-05-12 22:32:58]

The CQ bit was checked by tsepez@chromium.org

[commit-bot: I haz the power, 2016-05-12 22:33:01]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1975983002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1975983002/1

[commit-bot: I haz the power, 2016-05-12 22:49:58]

Description was changed from

==========
Fix comment in fx_string.h

Previous CLs have removed or marked explicit the constructors
which allowed bad constructs to compile.  I now get an error:
candidate constructor not viable: expects an l-value for 1st argument
when called as indicated in the comment.
==========

to

==========
Fix comment in fx_string.h

Previous CLs have removed or marked explicit the constructors
which allowed bad constructs to compile.  I now get an error:
candidate constructor not viable: expects an l-value for 1st argument
when called as indicated in the comment.

Committed:
https://pdfium.googlesource.com/pdfium/+/ade9465067098d9f94a13f61741cebf4bb8a...
==========

[commit-bot: I haz the power, 2016-05-12 22:50:01]

Committed patchset #1 (id:1) as
https://pdfium.googlesource.com/pdfium/+/ade9465067098d9f94a13f61741cebf4bb8a...