[FIX] web_editor: make the grid item drop more robust

It happened previously that the grid item could be broken if `top` and
`left` style properties were not set on it, which resulted in `NaN`
values being used in the grid-area computation.
This commit makes the drop more robust by obtaining their values through
the computed style instead of the style. This way these properties are
always defined.

task-3554752

closes odoo/odoo#143903

X-original-commit: 7a8fd5792f9eedfebf812b15730259de2b511a58
Signed-off-by: Soukéina Bojabza (sobo) <sobo@odoo.com>

addons/web_editor/static/src/js/editor/snippets.editor.js

@@ -1358,8 +1358,9 @@ var SnippetEditor = Widget.extend({
             // Defining the column grid area with its position.
             const gridProp = gridUtils._getGridProperties(rowEl);
 
-            const top = parseFloat(this.$target[0].style.top);
-            const left = parseFloat(this.$target[0].style.left);
+            const style = window.getComputedStyle(this.$target[0]);
+            const top = parseFloat(style.top);
+            const left = parseFloat(style.left);
 
             const rowStart = Math.round(top / (gridProp.rowSize + gridProp.rowGap)) + 1;
             const columnStart = Math.round(left / (gridProp.columnSize + gridProp.columnGap)) + 1;