[Chromecast] Use appropriate epsilon for audio unittests

The sum of the values being compared can be as large as 2.0, so epsilon
should be scaled accordingly.

Bug: internal b/177118785
Change-Id: I6ed39219dddfc5226383e4664937bebc18dcdaed
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2626925
Commit-Queue: Kenneth MacKay <kmackay@chromium.org>
Commit-Queue: Yuchen Liu <yucliu@chromium.org>
Auto-Submit: Kenneth MacKay <kmackay@chromium.org>
Reviewed-by: Yuchen Liu <yucliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843153}

chromecast/media/cma/backend/mixer/post_processors/post_processor_unittest.cc

@@ -21,7 +21,7 @@ namespace post_processor_test {
 
 namespace {
 
-const float kEpsilon = std::numeric_limits<float>::epsilon();
+const float kEpsilon = std::numeric_limits<float>::epsilon() * 2;
 
 // Benchmark parameters.
 const float kTestDurationSec = 1.0;