[Code Coverage] Use platform name for the root directory instead of 'coverage'.

That single-line change would be very helpful for hosting reports at GCS, as it
provides a multi-platform dir structure for free.

Bug: 836663
Change-Id: Ic9a984360cb352a23ace780d4aa600084be98a36
Reviewed-on: https://chromium-review.googlesource.com/1041282Reviewed-by: Abhishek Arya <inferno@chromium.org>
Commit-Queue: Abhishek Arya <inferno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555664}

tools/code_coverage/coverage.py

@@ -743,7 +743,7 @@ def _GetCoverageHtmlReportPathForComponent(component_name):
 
 def _GetCoverageReportRootDirPath():
   """The root directory that contains all generated coverage html reports."""
-  return os.path.join(os.path.abspath(OUTPUT_DIR), 'coverage')
+  return os.path.join(os.path.abspath(OUTPUT_DIR), _GetHostPlatform())
 
 
 def _CreateCoverageProfileDataForTargets(targets, commands, jobs_count=None):