Suppress one threadsafe_fn error and enable lint.

This CL suppresses the threadsafe_fn lint in the sample code and enables
the lint check.

Bug: dawn:1339
Change-Id: I51f6808b894ba1fcb7f3cdbbc274b35cd393dae8
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/86864
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>
Auto-Submit: Dan Sinclair <dsinclair@chromium.org>

src/dawn/CPPLINT.cfg

@@ -7,4 +7,3 @@ filter=-readability/todo
 filter=-runtime/explicit
 filter=-runtime/indentation_namespace
 filter=-runtime/int
-filter=-runtime/threadsafe_fn

src/dawn/samples/Animometer.cpp

@@ -31,6 +31,7 @@ wgpu::BindGroup bindGroup;
 wgpu::Buffer ubo;
 
 float RandomFloat(float min, float max) {
+    // NOLINTNEXTLINE(runtime/threadsafe_fn)
     float zeroOne = rand() / float(RAND_MAX);
     return zeroOne * (max - min) + min;
 }