Speculative hardening of fullscreen infobar code.

Speculative hardening of fullscreen infobar code.

While this probably isn't the principle infobar crash, it does have a
few bad smells:
1) InfoBarDelegate's already have an infobar() accessor. Use that
instead of a cached tab instance.
2) Many places in infobar code that handle user events check the
context to ensure they're still valid before handling them but this
didn't.
BUG=481758
TBR=acleung

Review URL: https://codereview.chromium.org/1322063003

Cr-Commit-Position: refs/heads/master@{#347017}
(cherry picked from commit 4d978263c720d2accbf1ffa35f859bee9f8e3383)

Committed: https://chromium.googlesource.com/chromium/src/+/44470c6d4b00c04521dc804bfa8a6f88a09cf1e4

[Yaron, 2015-09-11 20:51:22]

Committed patchset #1 (id:1) manually as
44470c6d4b00c04521dc804bfa8a6f88a09cf1e4 (presubmit successful).