[DMC #21] Add check to make TextMatchMarker cast safer

Add check to make TextMatchMarker cast safer

Currently, ToTextMatchMarker() is an unchecked cast. This CL adds a condition
for describing when this cast is safe so this cast can be DCHECKed in debug
mode.

BUG=707867
Review-Url: https://codereview.chromium.org/2904123003
Cr-Commit-Position: refs/heads/master@{#475112}
Committed: https://chromium.googlesource.com/chromium/src/+/4c1c503b1d3d6a8f4f7fcfce84796a0877255d1d

[rlanday, 2017-05-25 22:45:59]

rlanday@chromium.org changed reviewers:
+ xiaochengh@chromium.org, yosin@chromium.org

[yosin_UTC9, 2017-05-26 04:46:59]

lgtm

Please commit this patch before the patch using ToTextMatchMarker().

[rlanday, 2017-05-26 18:49:50]

Note: until https://codereview.chromium.org/2905753002, this check can pass and
the cast still might not be safe (if the TextMatchMarker hasn't been constructed
yet)...but it's still an improvement over wheat we currently have, so I'm just
going to land it

[rlanday, 2017-05-26 18:49:56]

The CQ bit was checked by rlanday@chromium.org

[rlanday, 2017-05-26 18:49:56]

The patchset sent to the CQ was uploaded after l-g-t-m from yosin@chromium.org
Link to the patchset: https://codereview.chromium.org/2904123003/#ps20001
(title: "Remove dep")

[commit-bot: I haz the power, 2017-05-26 18:50:35]

CQ is trying da patch.

Follow status at:
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2017-05-26 20:37:03]

CQ is committing da patch.

Bot data: {"patchset_id": 20001, "attempt_start_ts": 1495824596562680,
"parent_rev": "0d700dd8b90fd715b2355ad69e5fadfd4fa9eb27", "commit_rev":
"4c1c503b1d3d6a8f4f7fcfce84796a0877255d1d"}

[commit-bot: I haz the power, 2017-05-26 20:37:12]

Description was changed from

==========
Add check to make TextMatchMarker cast safer

Currently, ToTextMatchMarker() is an unchecked cast. This CL adds a condition
for describing when this cast is safe so this cast can be DCHECKed in debug
mode.

BUG=707867
==========

to

==========
Add check to make TextMatchMarker cast safer

Currently, ToTextMatchMarker() is an unchecked cast. This CL adds a condition
for describing when this cast is safe so this cast can be DCHECKed in debug
mode.

BUG=707867

Review-Url: https://codereview.chromium.org/2904123003
Cr-Commit-Position: refs/heads/master@{#475112}
Committed:
https://chromium.googlesource.com/chromium/src/+/4c1c503b1d3d6a8f4f7fcfce8479...
==========

[commit-bot: I haz the power, 2017-05-26 20:37:18]

Committed patchset #2 (id:20001) as
https://chromium.googlesource.com/chromium/src/+/4c1c503b1d3d6a8f4f7fcfce8479...