Reland "[chromium] Enable ChromeDriver log for WPT tests"

This is a reland of cf0f02ca
This CL failed originally not because of the step for generating
chromedriver log but for merging the logs from different shards.
This is the same CL as the reverted one except that I have
submitted a CL https://chromium-review.googlesource.com/c/chromium/src/+/2402319 in https://bugs.chromium.org/p/chromium/issues/detail?id=1127106#c7 to fix merge script
merging chromedriver log as json issue.

Original change's description:
> [chromium] Enable ChromeDriver log for WPT tests
>
> Enable chromedriver log for CI wpt tests
>
> Change-Id: I02d4acb627ca90233b7c0dab401c7c28e6759604
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2402319
> Reviewed-by: John Chen <johnchen@chromium.org>
> Reviewed-by: Scott Violet <sky@chromium.org>
> Commit-Queue: Shengfa Lin <shengfa@google.com>
> Cr-Commit-Position: refs/heads/master@{#805817}

Change-Id: Iff9fd780926dfbd277fa2b9d3b86dacf6ceeef94
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2406548Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: John Chen <johnchen@chromium.org>
Commit-Queue: Shengfa Lin <shengfa@google.com>
Cr-Commit-Position: refs/heads/master@{#806834}

testing/buildbot/gn_isolate_map.pyl

@@ -529,6 +529,7 @@
       "../../chrome/test/chromedriver/test/run_webdriver_tests.py",
       "-v",
       "--chromedriver=chromedriver",
+      "--log-path=${ISOLATED_OUTDIR}/chromedriver.log",
       "--output-dir=${ISOLATED_OUTDIR}",
       "--test-path=../../third_party/blink/web_tests/external/wpt/webdriver/tests/",
     ],