Chromium change:

https://chromium.googlesource.com/chromium/src/+/7a4dcba18df43e3200a37f4906c10348e8755148

commit 7a4dcba18df43e3200a37f4906c10348e8755148
Author: Takashi Toyoshima <toyoshim@chromium.org>
Date:   Mon Jul 22 11:28:27 2019 +0000

    Reland "mojo: Introduce StringDataSource to replace StringDataPipeProducer"

    This is a reland of 06c0387de9e47f4dcfff928b232a13854f128e3a

    Updates from the last patchset:
     - RunUntilIdle() in the SignedExchangeLoaderTest.Simple is replaced
      with QuitClosure() and Run() to fix the flakiness.

    Original change's description:
    > mojo: Introduce StringDataSource to replace StringDataPipeProducer
    >
    > This patch introduces StringDataSource to reuse DataPipeProducer
    > even for string uses.
    >
    > Now DataPipeProducer::DataSource has GetLength() interface so to
    > reduce unnecessary memory consumption for data pipe buffer.
    >
    > Bug: 983023
    > Change-Id: I03c783e571e3d75f0d6bdc19f6623c4da29853b8
    > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1699526
    > Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
    > Reviewed-by: Ken Rockot <rockot@google.com>
    > Commit-Queue: Takashi Toyoshima <toyoshim@chromium.org>
    > Auto-Submit: Takashi Toyoshima <toyoshim@chromium.org>
    > Cr-Commit-Position: refs/heads/master@{#678606}

    Bug: 983023