## [SIEM] Update Timeline to use the latest euiFlyoutBody style

[A recent update to EUI](elastic/eui#2837) requires an update to Timeline to restore its original padding, per the before / after screenshots below:

### Before

The previous code, which was using the `euiFlyoutBody__overflow` style, has extra padding (before this PR):

![timeline-padding-before-chrome](https://user-images.githubusercontent.com/4459398/76060425-81e37500-5f3e-11ea-8083-a58297afee85.png)

### After Chrome `80.0.3987.132`

The new code in this PR uses `euiFlyoutBody__overflowContent`, which restores the Timeline's original padding:

![timeline-paddng-after-chrome-default-theme](https://user-images.githubusercontent.com/4459398/76060436-8b6cdd00-5f3e-11ea-80d6-9ff0e74722c3.png)

![timeline-padding-after-chrome-dark-theme](https://user-images.githubusercontent.com/4459398/76060601-dedf2b00-5f3e-11ea-8a46-1008d7defe09.png)

### After Firefox `73.0.1`

![timeline-padding-after-firefox](https://user-images.githubusercontent.com/4459398/76060692-18b03180-5f3f-11ea-8277-31b6f9dbf211.png)

### After Safari `13.0.5`

![timeline-padding-after-safari](https://user-images.githubusercontent.com/4459398/76060744-38475a00-5f3f-11ea-9b04-cdd45fa41702.png)

x-pack/legacy/plugins/siem/public/components/flyout/pane/index.tsx

@@ -49,7 +49,7 @@ const EuiFlyoutContainer = styled.div<{ headerHeight: number }>`
   .timeline-flyout-body {
     overflow-y: hidden;
     padding: 0;
-    .euiFlyoutBody__overflow {
+    .euiFlyoutBody__overflowContent {
       padding: 0;
     }
   }