Support MacBook trackpad pinch to zoom gesture

[bbondy]

Test plan

Try trackpad pinch to zoom

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[luixxiul]

+1 from support

[bbondy]

cc @darkdh (but treat form fill and import data as higher priority in general)

[alexwykoff]

New +1 from support (https://linkbubble.zendesk.com/agent/tickets/5149)

[jkup]

Also double-tap to zoom being requested!

[alexwykoff]

+1 from support : https://linkbubble.zendesk.com/agent/tickets/6321

[cndouglas]

Double-tap to zoom is being tracked separately at #4451.

[alexwykoff]

+1 from support : https://linkbubble.zendesk.com/agent/tickets/6406

[iandundas]

+1 (seems doing +1 is acceptable here! apologies if not.. )

[srirambv]

+1 from community https://community.brave.com/t/pinch-and-spread-to-zoom/862

[jonathansampson]

+1 from user https://gab.ai/AquinasJohnPaul/posts/6373069

[jonathansampson]

+1 from Twitter user: https://twitter.com/schofeld/status/848594205463240705

Jonathan Schofield (@schofeld)
No pinch and zoom in @brave on iOS it seems. Please fix. #a11y

[dskrainka]

+1

[eljuno]

+1 from community

[b4rton]

+1 +1 +1 Basic browser functionality on Mac. Built into Chromium, no? Please add this.

[jandahl]

+1 - one of the most basic things for Mac users is missing.

[bsclifton]

Per discussion with @darkdh, we get this functionality with Chromium 61 😄 Let's create a new issue to track bugginess or things which could be better