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Setting the pan attribute will set the value immediately instead of
dezippering from the current value to the new value.

Chromium Feature: https://www.chromestatus.com/features/5287995770929152
Intent to ship: https://groups.google.com/a/chromium.org/d/msg/blink-dev/YKYRrh0nWMo/aGzd3049AgAJ

Bug: 752985
Test: StereoPanner/dezipper.html
Change-Id: I13815d7cb3a6d4ea93dd8f0f1bab739e6768e561
Reviewed-on: https://chromium-review.googlesource.com/607028
Commit-Queue: Raymond Toy <rtoy@chromium.org>
Reviewed-by: Hongchan Choi <hongchan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532527}