Relabeling button for a11y

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/b6f2815d8fafb4c720961ecb479ea5f156192eca

commit b6f2815d8fafb4c720961ecb479ea5f156192eca
Author: Hector Carmona <hcarmona@chromium.org>
Date: Tue Nov 14 03:29:35 2017

MD-Extensions: A11y - Aria label 'Update' to 'Update extensions now'.

When using only a screen reader, the 'Update' button loses context to
what it does. Adding a separate a11y label makes this clear for users on
screen readers.

Bug:  779264 
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I175524196aa818e2fb50acf6e5836bbabab078c0
Reviewed-on: https://chromium-review.googlesource.com/764600
Commit-Queue: Hector Carmona <hcarmona@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#516171}
[modify] https://crrev.com/b6f2815d8fafb4c720961ecb479ea5f156192eca/chrome/app/md_extensions_strings.grdp
[modify] https://crrev.com/b6f2815d8fafb4c720961ecb479ea5f156192eca/chrome/browser/resources/md_extensions/toolbar.html
[modify] https://crrev.com/b6f2815d8fafb4c720961ecb479ea5f156192eca/chrome/browser/ui/webui/extensions/extensions_ui.cc

Tested this issue on Windows 7,Mac 10.12.6 & ubuntu 14.04 using chrome latest Canary-64.0.3269.0 as per C#0 & C#4.

On Mac:[ Working as intended]
-------
1. launch chrome
2. Enabled md-extensions flag in chrome://flags
3. Navigate to chrome://extensions
4. Enable 'Developer mode' & observed 'Update' button
5. We can see 'Update extensions now' label When user hover over 'update' button
5. Open 'Voice over' application
6. Press tab to navigate to 'Update' button
7. Voice over is reading 'Update extensions now' label when focus is on 'Update' button


On Windows 7 & Ubuntu :[Chromevox is not reading]
---------------------
1. launch chrome
2. Enabled md-extensions flag in chrome://flags
3. Navigate to chrome://extensions
4. Enable 'Developer mode' & observed 'Update' button
5. We can see 'Update extensions now' label When user hover over 'update' button
5. Install 'Chromevox' extension
6. Press tab to navigate to 'Update' button
7. Chromevox is not reading 'Update extensions now' label When user hover over 'update' button

Please find the attached screencast for reference.

Thanks..!

Deleted: 779264-Mac-M64.mp4 1.3 MB

	779264-Mac-M64.mp4 1.3 MB View Download