Disable back button when stack is empty

Disable back button when stack is empty

If the back stack is empty
- The back button icon's color is that of the foreground with an alpha of 0x33
- Hovering over the back button doesn't change its color

Note:
- tested manually due to time constraints, but we should add a tests

BUG=726889
Review-Url: https://codereview.chromium.org/2906403002
Cr-Original-Commit-Position: refs/heads/master@{#475375}
Review-Url: https://codereview.chromium.org/2912953002 .
Cr-Commit-Position: refs/branch-heads/3112@{#26}
Cr-Branched-From: b6460e24cf59f429d69de255538d0fc7a425ccf9-refs/heads/master@{#474897}
Committed: https://chromium.googlesource.com/chromium/src/+/e8cc130f7fa4718d1656d00276f3303ce48e9393

[Ian Vollick, 2017-05-30 12:53:54]

Description was changed from

==========
Disable back button when stack is empty

If the back stack is empty
- The back button icon's color is that of the foreground with an alpha of 0x33
- Hovering over the back button doesn't change its color

Note:
- tested manually due to time constraints, but we should add a tests

BUG=726889

Review-Url: https://codereview.chromium.org/2906403002
Cr-Commit-Position: refs/heads/master@{#475375}
(cherry picked from commit ed4df89fed450d8f2741bba7cd2651ad28c6a779)
==========

to

==========
Disable back button when stack is empty

If the back stack is empty
- The back button icon's color is that of the foreground with an alpha of 0x33
- Hovering over the back button doesn't change its color

Note:
- tested manually due to time constraints, but we should add a tests

BUG=726889

Review-Url: https://codereview.chromium.org/2906403002
Cr-Original-Commit-Position: refs/heads/master@{#475375}
Review-Url: https://codereview.chromium.org/2912953002 .
Cr-Commit-Position: refs/branch-heads/3112@{#26}
Cr-Branched-From:
b6460e24cf59f429d69de255538d0fc7a425ccf9-refs/heads/master@{#474897}
Committed:
https://chromium.googlesource.com/chromium/src/+/e8cc130f7fa4718d1656d00276f3...
==========

[Ian Vollick, 2017-05-30 12:53:56]

Committed patchset #1 (id:1) manually as
e8cc130f7fa4718d1656d00276f3303ce48e9393.