The previous CL was reverted because it broke layout tests.

Cause:
GetKeyModifiersFromV8() in test_runner/event_sender.cc passes
NULL as v8::Isolate*, which is cleary wrong. Pass
blink::mainThreadIsolate() as the other callsites do.

BUG=479439

Review URL: https://codereview.chromium.org/1152653004

Cr-Commit-Position: refs/heads/master@{#332312}