[tools] Strip path from library when applying targetRootFS in tickprocessor

This allows us to use this flag to specify the local location of a
shared library when processing a trace run on Android.

Change-Id: I3aa44beaed700b8c5354af12a4df176aa1cbfd0c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1681137
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
Auto-Submit: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62448}

tools/tickprocessor.js

@@ -722,7 +722,10 @@ UnixCppEntriesProvider.prototype.loadSymbols = function(libName) {
   if (this.apkEmbeddedLibrary && libName.endsWith('.apk')) {
     libName = this.apkEmbeddedLibrary;
   }
+  if (this.targetRootFS) {
+    libName = libName.substring(libName.lastIndexOf('/') + 1);
     libName = this.targetRootFS + libName;
+  }
   try {
     this.symbols = [
       os.system(this.nmExec, ['-C', '-n', '-S', libName], -1, -1),