Convert WebTestHostMsg_GetWritableDirectory to Mojo

This CL migrates the legacy GetWritableDirectory sync IPC messages to
the new Mojo defined in WebTestClient interface.

Additionally, this CL removes LocalFileToDataURL and ReadFileToString
because nobody uses it.

Bug: 1039247
Change-Id: Iecc34334d67e77b1a161da671a1a4f36d4113f8a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2038912
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Commit-Queue: Gyuyoung Kim <gyuyoung@igalia.com>
Cr-Commit-Position: refs/heads/master@{#741035}