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A common pattern for converting base::Optional<T> to T* is:
  optional ? &*optional : nullptr
This patch adds a helper to make the pattern simpler: OptionalOrNullptr.
This expression is relatively common, in part, because the styleguide
recommends T* parameters over Optional<T>:
https://chromium.googlesource.com/chromium/src/+/master/docs/optional.md#When-not-to-use

This patch also updates callsites in third_party/WebKit/* and cc/*.

Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: I2c8b31b52480ad69228123df951486634edec006
Reviewed-on: https://chromium-review.googlesource.com/950226
Commit-Queue: Philip Rogers <pdr@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541993}