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In renderer to browser messages, this field isn't used and, because
WTF::String supports nullability, the value was serialized as null.

In addition, this CL adds and fixes some of the checks done in the
DragData Mojo type mapping.

Bug: 1136901
Change-Id: Ifc9f9343817d2e06d734470573ce3785cbe885d3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2489897
Commit-Queue: Henrique Ferreiro <hferreiro@igalia.com>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821179}