ReadFile returned a Vector with ownership transfer, i.e. the client
needed to call Dispose to free the memory. This CL changes the interface
to return a std::string instead, which manages ownership. As it turns
out, there is only one user of ReadString that sometimes calls an API
function which expects to take ownership of its Vector argument.

Bug: v8:7932
Cq-Include-Trybots: luci.chromium.try:linux_chromium_headless_rel;luci.chromium.try:linux_chromium_rel_ng;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Ie624a7a65cf5814fddce7a57bc557e4b9876bc53
Reviewed-on: https://chromium-review.googlesource.com/1155115
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54805}