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The omnibox already had a reimplentation of GTMFadeTruncatingLabel
that used NSAttributedText property to perform the fade-truncation
as OmniboxPopupTruncatingLabel.

Move it to a shared location and rename it to FadeTruncatingLabel
and convert TabView to use the new class thus removing another
dependency on third_party/google_toolbox_for_mac.

Before: https://drive.google.com/file/d/1YeEL6jkm-P9cNeEgS0vJxlPu-HIEo3_r/view
After:  https://drive.google.com/file/d/1U1jAVeXYg4RFiw98UzZ-yoBBoOG2e1MD/view

Bug: none
Change-Id: Ic43f0973d36ba6b74c0ad0a2fa9068d5a3be9bb1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1901082
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713397}