Revert "Use high performance GPU by default in GPU tests."

This reverts commit 8d4643f0.

Reason for revert: caused some pixel and WebGL test failures per http://crbug.com/965842 .

Original change's description:
> Use high performance GPU by default in GPU tests.
> 
> Upcoming CLs will change the heuristics for selecting the low-power
> vs. high-performance GPUs on dual-GPU systems, macOS in particular.
> The GPU tests in Chromium's hardware fleet expect to run on the
> high-power GPU, so maintain this behavior by default.
> 
> Bug: 965842
> Change-Id: I17707475a4505905ea95a8a57d06a1754fc101fc
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1665910
> Reviewed-by: ccameron <ccameron@chromium.org>
> Commit-Queue: Kenneth Russell <kbr@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#670302}

TBR=ccameron@chromium.org,kbr@chromium.org

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

Bug: 965842
Change-Id: I6402b31efe05ebacb5a488c5d6cecba4f790c1db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1671863Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#671481}

content/test/gpu/gpu_tests/gpu_integration_test.py

@@ -86,13 +86,6 @@ class GpuIntegrationTest(
     browser_args.append(
       '--disable-gpu-process-for-dx12-vulkan-info-collection')
 
-    if not ('--force_low_power_gpu' in browser_args or
-            '--force_high_performance_gpu' in browser_args):
-      # Force the GPU tests to use the high-performance GPU on dual-GPU machines
-      # by default. In Chromium's hardware fleet this currently only has an
-      # effect on the dual-GPU MacBook Pros.
-      browser_args.append('--force_high_performance_gpu')
-
     # Append the new arguments.
     browser_options.AppendExtraBrowserArgs(browser_args)
     cls._last_launched_browser_args = set(browser_args)