Revert "[code coverage] Change log warning to info"

This reverts commit 5b3adc3c.

Reason for revert: This turns out to be NOT helpful to debugging the issue.

Original change's description:
> [code coverage] Change log warning to info
> 
> Will ssh to bot if still nothing printed.
> 
> Bug: 1027818
> Change-Id: I98147da43689d2b4ea9db0ee8643312eac6fc72b
> No-Try: true
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1935860
> Auto-Submit: Yun Liu <yliuyliu@google.com>
> Commit-Queue: Yuke Liao <liaoyuke@chromium.org>
> Reviewed-by: Yuke Liao <liaoyuke@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#719180}

TBR=liaoyuke@chromium.org,yliuyliu@google.com

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 1027818
Change-Id: Ibc49a3f10e3847a1b269e2f501ba321ef943b9a7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1948489Reviewed-by: Yuke Liao <liaoyuke@chromium.org>
Reviewed-by: Yun Liu <yliuyliu@google.com>
Commit-Queue: Yuke Liao <liaoyuke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#721178}

testing/merge_scripts/code_coverage/merge_lib.py

@@ -179,7 +179,7 @@ def _validate_and_convert_profraw(profraw_file, output_profdata_files,
     else:
       profile_valid = True
   except subprocess.CalledProcessError as error:
-    logging.info('Validating and converting %r to %r failed with output: %r',
+    logging.warning('Validating and converting %r to %r failed with output: %r',
                     profraw_file, output_profdata_file, error.output)
     validation_output = error.output