Add more information to cannot find Chrome message (#41695)

packages/flutter_tools/lib/src/web/web_validator.dart

@@ -22,7 +22,8 @@ class WebValidator extends DoctorValidator {
           ValidationMessage('$kChromeEnvironment = $chrome')
       else
         if (!canRunChrome)
-          ValidationMessage.hint('$kChromeEnvironment not set')
+          ValidationMessage.hint('Cannot find Chrome. Try setting '
+            '$kChromeEnvironment to a Chrome executable.')
         else
           ValidationMessage('Chrome at $chrome'),
     ];

packages/flutter_tools/test/general.shard/web/web_validator_test.dart

@@ -50,7 +50,7 @@ void main() {
       when(mockProcessManager.canRun(kMacOSExecutable)).thenReturn(false);
       final ValidationResult result = await webValidator.validate();
       expect(result.messages, <ValidationMessage>[
-        ValidationMessage.hint('CHROME_EXECUTABLE not set'),
+        ValidationMessage.hint('Cannot find Chrome. Try setting CHROME_EXECUTABLE to a Chrome executable.'),
       ]);
       expect(result.type, ValidationType.missing);
     }));