[ChromeDriver] Fix takeHeapSnapshot command

Return heap snapshot as a string instead of JSON object.
This resolves two issues:
* The snapshot sometimes uses characters not allowed by our JSON parser.
* Chrome's snapshot reader is sensitive to the ordering of properties
  in the snapshot. Converting the snapshot into JSON object and then
  re-serialize into string often results in a file not loadable by the
  snapshot reader.

Bug: chromedriver:2964, chromedriver:1616
Change-Id: Ia721c28ed78a085d567111459c9e50d8bc26f81d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1679224
Reviewed-by: John Chen <johnchen@chromium.org>
Commit-Queue: Tatiana Buldina <buldina@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672666}

• chrome/test/chromedriver/chrome/heap_snapshot_taker.cc[diff]
• chrome/test/chromedriver/chrome/heap_snapshot_taker_unittest.cc[diff]