Merge background.js and sw.js into bg.js

Now that we have included webpack-preprocessor-loader into Webpack, we
can merge the two files and hide code behind conditional statements
which will be preprocessed at compile time.

Change-Id: I99c87ca33fcb231d04c51ff9651e0cf10443e60b
diff --git a/src/common/actionApi.js b/src/common/actionApi.js
new file mode 100644
index 0000000..69c68b8
--- /dev/null
+++ b/src/common/actionApi.js
@@ -0,0 +1,5 @@
+// #!if browser_target == 'chromium_mv3'
+export default chrome.action;
+// #!else
+export default chrome.browserAction;
+// #!endif