[ios] Log UMA if didStartProvisionalNavigation doesn't add pending item.

[ios] Log UMA if didStartProvisionalNavigation doesn't add pending item.

Chrome for iOS crashes in |didCommitNavigation:| with empty Navigation
Manager. It's unclear how app can get into this state and one assumption
is that didStartProvisionalNavigation: does not add a pending navigation
item. This UMA histogram is logged when didStartProvisionalNavigation:
fails to add a pending item.

This histogram is added only to verify the assumption and should be removed
after M46 is released.

BUG=676458
Review-Url: https://codereview.chromium.org/2602133002
Cr-Commit-Position: refs/heads/master@{#442047}
(cherry picked from commit e6fef8190db76bbc5e9716d46296bfda636fad7f)

Review-Url: https://codereview.chromium.org/2616183002 .
Cr-Commit-Position: refs/branch-heads/2924@{#690}
Cr-Branched-From: 3a87aecc31cd1ffe751dd72c04e5a96a1fc8108a-refs/heads/master@{#433059}
Committed: https://chromium.googlesource.com/chromium/src/+/424c4cbf214daef4fc33ec5e3c33fb0146a04992

[Eugene But (OOO till 7-30), 2017-01-06 23:39:35]

Description was changed from

==========
[ios] Log UMA if didStartProvisionalNavigation doesn't add pending item.

Chrome for iOS crashes in |didCommitNavigation:| with empty Navigation
Manager. It's unclear how app can get into this state and one assumption
is that didStartProvisionalNavigation: does not add a pending navigation
item. This UMA histogram is logged when didStartProvisionalNavigation:
fails to add a pending item.

This histogram is added only to verify the assumption and should be removed
after M46 is released.

BUG=676458

Review-Url: https://codereview.chromium.org/2602133002
Cr-Commit-Position: refs/heads/master@{#442047}
(cherry picked from commit e6fef8190db76bbc5e9716d46296bfda636fad7f)
==========

to

==========
[ios] Log UMA if didStartProvisionalNavigation doesn't add pending item.

Chrome for iOS crashes in |didCommitNavigation:| with empty Navigation
Manager. It's unclear how app can get into this state and one assumption
is that didStartProvisionalNavigation: does not add a pending navigation
item. This UMA histogram is logged when didStartProvisionalNavigation:
fails to add a pending item.

This histogram is added only to verify the assumption and should be removed
after M46 is released.

BUG=676458

Review-Url: https://codereview.chromium.org/2602133002
Cr-Commit-Position: refs/heads/master@{#442047}
(cherry picked from commit e6fef8190db76bbc5e9716d46296bfda636fad7f)

Review-Url: https://codereview.chromium.org/2616183002 .
Cr-Commit-Position: refs/branch-heads/2924@{#690}
Cr-Branched-From:
3a87aecc31cd1ffe751dd72c04e5a96a1fc8108a-refs/heads/master@{#433059}
Committed:
https://chromium.googlesource.com/chromium/src/+/424c4cbf214daef4fc33ec5e3c33...
==========

[Eugene But (OOO till 7-30), 2017-01-06 23:39:36]

Committed patchset #1 (id:1) manually as
424c4cbf214daef4fc33ec5e3c33fb0146a04992.