Update SuggestionTextColour to kGoogleGrey700

To meet Accessibility's contrast requirement of 4.5:1 minimum.
Using kGoogleGrey700, contrast should be 5.91

Screenshot view: https://screenshot.googleplex.com/8oODpaXk02Y

Bug: 1042084
Change-Id: I7c9980527fe915cb30a34a83f100073d0db4030b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2224424Reviewed-by: Keith Lee <keithlee@chromium.org>
Commit-Queue: My Nguyen <myy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#774455}

ui/chromeos/ime/suggestion_view.h

@@ -34,8 +34,7 @@ constexpr int kPadding = 10;
 constexpr int kAnnotationPaddingHeight = 6;
 constexpr char kTabKey[] = "tab";
 constexpr SkColor kConfirmedTextColor = gfx::kGoogleGrey900;
-constexpr SkColor kSuggestionColor =
-    SkColorSetA(gfx::kGoogleGrey900, gfx::kGoogleGreyAlpha500);
+constexpr SkColor kSuggestionColor = gfx::kGoogleGrey700;
 
 // SuggestionView renders a suggestion.
 class UI_CHROMEOS_EXPORT SuggestionView : public views::View {