Do NULL checks on TabAndroids retrieved from the model
Do NULL checks on TabAndroids retrieved from the model

There can be TabModels which know about Tabs that are not
alive and cannot yet be instantiated.  Do NULL checks before
checking the Tabs.

BUG=403592
TBR=dgozman@chromium.org, dtrainor@chromium.org

>>> Cherry-picked from master to 2125.

Review URL: https://codereview.chromium.org/542953002

Cr-Commit-Position: refs/heads/master@{#293574}

Review URL: https://codereview.chromium.org/565303002

Cr-Commit-Position: 0d344d3782ceeadf0e8e8be3f4ce70193b218c54@{#329}
Cr-Branched-From: 2cb3f0147827a1600e8cbbcc52ca24c0febcb7a4@{#290040}

ondra-novak/chromium.src,littlstar/chromium.src,littlstar/chromium.src,littlstar/chromium.src,bright-sparks/chromium-spacewalk,ondra-novak/chromium.src,bright-sparks/chromium-spacewalk,ondra-novak/chromium.src,bright-sparks/chromium-spacewalk,bright-sparks/chromium-spacewalk,littlstar/chromium.src,littlstar/chromium.src,bright-sparks/chromium-spacewalk,ondra-novak/chromium.src,ondra-novak/chromium.src,ondra-novak/chromium.src,bright-sparks/chromium-spacewalk,ondra-novak/chromium.src,littlstar/chromium.src,bright-sparks/chromium-spacewalk,ondra-novak/chromium.src,littlstar/chromium.src,bright-sparks/chromium-spacewalk,ondra-novak/chromium.src,bright-sparks/chromium-spacewalk,littlstar/chromium.src