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When right click on an extension browser action in the Omnibox, the contextual
drop down says "Manage", which is unclear. Make the string to be more specific
such as "Manage Extensions".
The string's casing matches the platform; i.e. "Manage Extensions" for Mac,
"Manage extensions" everywhere else.

grd file changes:
Modifying 1 line label and 1 line desc text each in title case and in non title
case responsible for the label in: chrome/app/generated_resources.grd

BUG=418691
R=rouslan@chromium.org

TEST=
1) Launch chrome and install few extensions.
2) Right click an extension and observe
3) The context menu option for managing extensions should be string
   "Manage Extensions" on MAC, "Manage extensions" everywhere else.

Committed: https://crrev.com/4b4b173612017d0e45c2be082fb6f23baa79dc2a
Cr-Commit-Position: refs/heads/master@{#300516}

Review URL: https://codereview.chromium.org/659663004

Cr-Commit-Position: refs/heads/master@{#300673}