Move SystemLogsSource class into new file

Splits SystemLogsSource out of system_logs_fetcher.h.

BUG=710168

Review-Url: https://codereview.chromium.org/2969573002
Cr-Commit-Position: refs/heads/master@{#484674}
diff --git a/chrome/browser/BUILD.gn b/chrome/browser/BUILD.gn
index 19e685be..ae8988c 100644
--- a/chrome/browser/BUILD.gn
+++ b/chrome/browser/BUILD.gn
@@ -3182,6 +3182,8 @@
       "feedback/system_logs/log_sources/memory_details_log_source.h",
       "feedback/system_logs/system_logs_fetcher.cc",
       "feedback/system_logs/system_logs_fetcher.h",
+      "feedback/system_logs/system_logs_source.cc",
+      "feedback/system_logs/system_logs_source.h",
       "first_run/first_run.cc",
       "first_run/first_run.h",
       "first_run/first_run_dialog.h",