Fix name of GPU.GPUProcessSoftwareRendering histogram.

The histograms.xml file has the correct name, but apparently the source
code has always been reporting under an incorrect name.

BUG=643538

Review-Url: https://codereview.chromium.org/2302193003
Cr-Commit-Position: refs/heads/master@{#416378}

content/browser/gpu/gpu_process_host.cc

@@ -1032,7 +1032,7 @@ bool GpuProcessHost::LaunchGpuProcess(gpu::GpuPreferences* gpu_preferences) {
     return false;
   }
 
-  UMA_HISTOGRAM_BOOLEAN("GPU.GPU.GPUProcessSoftwareRendering",
+  UMA_HISTOGRAM_BOOLEAN("GPU.GPUProcessSoftwareRendering",
                         swiftshader_rendering_);
 
   // If specified, prepend a launcher program to the command line.