I believe that by adding icons to all of the menu items would make it hard to scan and hard to navigate.
Why is that? Well icons have meaning, unfortunately the meaning is not universal, people from different backgrounds and cultures would interpret the icons differently so instead of reading the menu and clicking a right menu item. myth-icons-enhance-usability
The user will be distracted with the array of icons and colours and then they have to scan the menu either for a familiar icon and then read the text next to it to make sure that they have chosen the right menu item, or they have to overlook the icons and go for text.
Also on my Chrome I have extensions like Pocket, Evernote and Stylebot installed and the icons are very helpful!
Chrome with a few icons:
Now take a look at what would happen if chrome by default would have their menu full of icons!
As you can see 2nd menu didn't make it any better actually I think it looks overly complicated, also if you look at items like "Translate to English", page source are almost impossible to understand without reading the text. Also try reading the menu, you will notice that icons are shouting and demand your attention.